cascade – CSS-Tricks

Taming the Cascade With BEM and Modern CSS Selectors

Liam Johnston — Mon, 21 Nov 2022 13:59:15 +0000

BEM. Like seemingly all techniques in the world of front-end development, writing CSS in a BEM format can be polarizing. But it is – at least in my Twitter bubble – one of the better-liked CSS methodologies.

Personally, I think BEM is good, and I think you should use it. But I also get why you might not.

Regardless of your opinion on BEM, it offers several benefits, the biggest being that it helps avoid specificity clashes in the CSS Cascade. That’s because, if used properly, any selectors written in a BEM format should have the same specificity score (0,1,0). I’ve architected the CSS for plenty of large-scale websites over the years (think government, universities, and banks), and it’s on these larger projects where I’ve found that BEM really shines. Writing CSS is much more fun when you have confidence that the styles you’re writing or editing aren’t affecting some other part of the site.

There are actually exceptions where it is deemed totally acceptable to add specificity. For instance: the :hover and :focus pseudo classes. Those have a specificity score of 0,2,0. Another is pseudo elements — like ::before and ::after — which have a specificity score of 0,1,1. For the rest of this article though, let’s assume we don’t want any other specificity creep. 🤓

But I’m not really here to sell you on BEM. Instead, I want to talk about how we can use it alongside modern CSS selectors — think :is(), :has(), :where(), etc. — to gain even more control of the Cascade.

What’s this about modern CSS selectors?

The CSS Selectors Level 4 spec gives us some powerful new(ish) ways to select elements. Some of my favorites include :is(), :where(), and :not(), each of which is supported by all modern browsers and is safe to use on almost any project nowadays.

:is() and :where() are basically the same thing except for how they impact specificity. Specifically, :where() always has a specificity score of 0,0,0. Yep, even :where(button#widget.some-class) has no specificity. Meanwhile, the specificity of :is() is the element in its argument list with the highest specificity. So, already we have a Cascade-wrangling distinction between two modern selectors that we can work with.

The incredibly powerful :has() relational pseudo-class is also rapidly gaining browser support (and is the biggest new feature of CSS since Grid, in my humble opinion). However, at time of writing, browser support for :has() isn’t quite good enough for use in production just yet.

Lemme stick one of those pseudo-classes in my BEM and…

/* ❌ specificity score: 0,2,0 */
.something:not(.something--special) {
  /* styles for all somethings, except for the special somethings */
}

Whoops! See that specificity score? Remember, with BEM we ideally want our selectors to all have a specificity score of 0,1,0. Why is 0,2,0 bad? Consider a similar example, expanded:

.something:not(a) {
  color: red;
}
.something--special {
  color: blue;
}

Even though the second selector is last in the source order, the first selector’s higher specificity (0,1,1) wins, and the color of .something--special elements will be set to red. That is, assuming your BEM is written properly and the selected element has both the .something base class and .something--special modifier class applied to it in the HTML.

Used carelessly, these pseudo-classes can impact the Cascade in unexpected ways. And it’s these sorts of inconsistencies that can create headaches down the line, especially on larger and more complex codebases.

Dang. So now what?

Remember what I was saying about :where() and the fact that its specificity is zero? We can use that to our advantage:

/* ✅ specificity score: 0,1,0 */
.something:where(:not(.something--special)) {
  /* etc. */
}

The first part of this selector (.something) gets its usual specificity score of 0,1,0. But :where() — and everything inside it — has a specificity of 0, which does not increase the specificity of the selector any further.

`:where()` allows us to nest

Folks who don’t care as much as me about specificity (and that’s probably a lot of people, to be fair) have had it pretty good when it comes to nesting. With some carefree keyboard strokes, we may wind up with CSS like this (note that I’m using Sass for brevity):

.card { ... }

.card--featured {
  /* etc. */  
  .card__title { ... }
  .card__title { ... }
}

.card__title { ... }
.card__img { ... }

In this example, we have a .card component. When it’s a “featured” card (using the .card--featured class), the card’s title and image needs to be styled differently. But, as we now know, the code above results in a specificity score that is inconsistent with the rest of our system.

A die-hard specificity nerd might have done this instead:

.card { ... }
.card--featured { ... }
.card__title { ... }
.card__title--featured { ... }
.card__img { ... }
.card__img--featured { ... }

That’s not so bad, right? Frankly, this is beautiful CSS.

There is a downside in the HTML though. Seasoned BEM authors are probably painfully aware of the clunky template logic that’s required to conditionally apply modifier classes to multiple elements. In this example, the HTML template needs to conditionally add the --featured modifier class to three elements (.card, .card__title, and .card__img) though probably even more in a real-world example. That’s a lot of if statements.

The :where() selector can help us write a lot less template logic — and fewer BEM classes to boot — without adding to the level of specificity.

.card { ... }
.card--featured { ... }

.card__title { ... }
:where(.card--featured) .card__title { ... }

.card__img { ... }
:where(.card--featured) .card__img { ... }

Here’s same thing but in Sass (note the trailing ampersands):

.card { ... }
.card--featured { ... }
.card__title { 
  /* etc. */ 
  :where(.card--featured) & { ... }
}
.card__img { 
  /* etc. */ 
  :where(.card--featured) & { ... }
}

Whether or not you should opt for this approach over applying modifier classes to the various child elements is a matter of personal preference. But at least :where() gives us the choice now!

What about non-BEM HTML?

We don’t live in a perfect world. Sometimes you need to deal with HTML that is outside of your control. For instance, a third-party script that injects HTML that you need to style. That markup often isn’t written with BEM class names. In some cases those styles don’t use classes at all but IDs!

Once again, :where() has our back. This solution is slightly hacky, as we need to reference the class of an element somewhere further up the DOM tree that we know exists.

/* ❌ specificity score: 1,0,0 */
#widget {
  /* etc. */
}

/* ✅ specificity score: 0,1,0 */
.page-wrapper :where(#widget) {
  /* etc. */
}

Referencing a parent element feels a little risky and restrictive though. What if that parent class changes or isn’t there for some reason? A better (but perhaps equally hacky) solution would be to use :is() instead. Remember, the specificity of :is() is equal to the most specific selector in its selector list.

So, instead of referencing a class we know (or hope!) exists with :where(), as in the above example, we could reference a made up class and the tag.

/* ✅ specificity score: 0,1,0 */
:is(.dummy-class, body) :where(#widget) {
  /* etc. */
}

The ever-present body will help us select our #widget element, and the presence of the .dummy-class class inside the same :is() gives the body selector the same specificity score as a class (0,1,0)… and the use of :where() ensures the selector doesn’t get any more specific than that.

That’s it!

That’s how we can leverage the modern specificity-managing features of the :is() and :where() pseudo-classes alongside the specificity collision prevention that we get when writing CSS in a BEM format. And in the not too distant future, once :has() gains Firefox support (it’s currently supported behind a flag at the time of writing) we’ll likely want to pair it with :where() to undo its specificity.

Whether you go all-in on BEM naming or not, I hope we can agree that having consistency in selector specificity is a good thing!

Taming the Cascade With BEM and Modern CSS Selectors originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

Don’t Fight the Cascade, Control It!

Mads Stoumann — Mon, 10 Jan 2022 15:22:08 +0000

If you’re disciplined and make use of the inheritance that the CSS cascade provides, you’ll end up writing less CSS. But because our styles often comes from all kinds of sources — and can be a pain to structure and maintain—the cascade can be a source of frustration, and the reason we end up with more CSS than necessary.

Some years ago, Harry Roberts came up with ITCSS and it’s a clever way of structuring CSS.

Mixed with BEM, ITCSS has become a popular way that people write and organize CSS.

However, even with ITCSS and BEM, there are still times where we still struggle with the cascade. For example, I’m sure you’ve had to @import external CSS components at a specific location to prevent breaking things, or reach for the dreaded !important at some point in time.

Recently, some new tools were added to our CSS toolbox, and they allow us to finally control the cascade. Let’s look at them.

O cascade, `:where` art thou?

Using the :where pseudo-selector allows us to remove specificity to “just after the user-agent default styles,” no matter where or when the CSS is loaded into the document. That means the specificity of the whole thing is literally zero — totally wiped out. This is handy for generic components, which we’ll look into in a moment.

First, imagine some generic

styles, using :where:

:where(table) {
  background-color: tan;
}

Now, if you add some other table styles before the :where selector, like this:

table {
  background-color: hotpink;
}

:where(table) {
  background-color: tan;
}

…the table background becomes hotpink, even though the table selector is specified before the :where selector in the cascade. That’s the beauty of :where, and why it’s already being used for CSS resets.

:where has a sibling, which has almost the exact opposite effect: the :is selector.

The specificity of the :is() pseudo-class is replaced by the specificity of its most specific argument. Thus, a selector written with :is() does not necessarily have equivalent specificity to the equivalent selector written without :is(). Selectors Level 4 specification

Expanding on our previous example:

:is(table) {
  --tbl-bgc: orange;
}
table {
  --tbl-bgc: tan;
}
:where(table) {
  --tbl-bgc: hotpink;
  background-color: var(--tbl-bgc);
}

The

background color will be tan because the specificity of :is is the same as table, but table is placed after.

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However, if we were to change it to this:

:is(table, .c-tbl) {
  --tbl-bgc: orange;
}

…the background color will be orange, since :is has the weight of it’s heaviest selector, which is .c-tbl.

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Example: A configurable table component

Now, let’s see how we can use :where in our components. We’ll be building a table component, starting with the HTML:

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Let’s wrap .c-tbl in a :where-selector and, just for fun, add rounded corners to the table. That means we need border-collapse: separate, as we can’t use border-radius on table cells when the table is using border-collapse: collapse:

:where(.c-tbl) {
  border-collapse: separate;
  border-spacing: 0;
  table-layout: auto;
  width: 99.9%;
}

The cells use different styling for the

and -cells:

:where(.c-tbl thead th) {
  background-color: hsl(200, 60%, 40%);
  border-style: solid;
  border-block-start-width: 0;
  border-inline-end-width: 1px;
  border-block-end-width: 0;
  border-inline-start-width: 0;
  color: hsl(200, 60%, 99%);
  padding-block: 1.25ch;
  padding-inline: 2ch;
  text-transform: uppercase;
}
:where(.c-tbl tbody td) {
  background-color: #FFF;
  border-color: hsl(200, 60%, 80%);
  border-style: solid;
  border-block-start-width: 0;
  border-inline-end-width: 1px;
  border-block-end-width: 1px;
  border-inline-start-width: 0;
  padding-block: 1.25ch;
  padding-inline: 2ch;
}

And, because of our rounded corners and the missing border-collapse: collapse, we need to add some extra styles, specifically for the table borders and a hover state on the cells:

:where(.c-tbl tr td:first-of-type) {
  border-inline-start-width: 1px;
}
:where(.c-tbl tr th:last-of-type) {
  border-inline-color: hsl(200, 60%, 40%);
}
:where(.c-tbl tr th:first-of-type) {
  border-inline-start-color: hsl(200, 60%, 40%);
}
:where(.c-tbl thead th:first-of-type) {
  border-start-start-radius: 0.5rem;
}
:where(.c-tbl thead th:last-of-type) {
  border-start-end-radius: 0.5rem;
}
:where(.c-tbl tbody tr:last-of-type td:first-of-type) {
  border-end-start-radius: 0.5rem;
}
:where(.c-tbl tr:last-of-type td:last-of-type) {
  border-end-end-radius: 0.5rem;
}
/* hover */
@media (hover: hover) {
  :where(.c-tbl) tr:hover td {
    background-color: hsl(200, 60%, 95%);
  }
}

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Now we can create variations of our table component by injecting other styles before or after our generic styles (courtesy of the specificity-stripping powers of :where), either by overwriting the .c-tbl element or by adding a BEM-style modifier-class (e.g. c-tbl--purple):

.c-tbl--purple th {
  background-color: hsl(330, 50%, 40%)
}
.c-tbl--purple td {
  border-color: hsl(330, 40%, 80%);
}
.c-tbl--purple tr th:last-of-type {
  border-inline-color: hsl(330, 50%, 40%);
}
.c-tbl--purple tr th:first-of-type {
  border-inline-start-color: hsl(330, 50%, 40%);
}

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Cool! But notice how we keep repeating colors? And what if we want to change the border-radius or the border-width? That would end up with a lot of repeated CSS.

Let’s move all of these to CSS custom properties and, while we’re at it, we can move all configurable properties to the top of the component’s “scope“ — which is the table element itself — so we can easily play around with them later.

CSS Custom Properties

I’m going to switch things up in the HTML and use a data-component attribute on the table element that can be targeted for styling.

That data-component will hold the generic styles that we can use on any instance of the component, i.e. the styles the table needs no matter what color variation we apply. The styles for a specific table component instance will be contained in a regular class, using custom properties from the generic component.

[data-component="table"] {
  /* Styles needed for all table variations */
}
.c-tbl--purple {
  /* Styles for the purple variation */
}

If we place all the generic styles in a data-attribute, we can use whatever naming convention we want. This way, we don’t have to worry if your boss insists on naming the table’s classes something like .BIGCORP__TABLE, .table-component or something else.

In the generic component, each CSS property points to a custom property. Properties, that have to work on child-elements, like border-color, are specified at the root of the generic component:

:where([data-component="table"]) {
  /* These will will be used multiple times, and in other selectors */
  --tbl-hue: 200;
  --tbl-sat: 50%;
  --tbl-bdc: hsl(var(--tbl-hue), var(--tbl-sat), 80%);
}

/* Here, it's used on a child-node: */
:where([data-component="table"] td) {
  border-color: var(--tbl-bdc);
}

For other properties, decide whether it should have a static value, or be configurable with its own custom property. If you’re using custom properties, remember to define a default value that the table can fall back to in the event that a variation class is missing.

:where([data-component="table"]) {
  /* These are optional, with fallbacks */
  background-color: var(--tbl-bgc, transparent);
  border-collapse: var(--tbl-bdcl, separate);
}

If you’re wondering how I’m naming the custom properties, I’m using a component-prefix (e.g. --tbl) followed by an Emmett-abbreviation (e.g. -bgc). In this case, --tbl is the component-prefix, -bgc is the background color, and -bdcl is the border collapse. So, for example, --tbl-bgc is the table component’s background color. I only use this naming convention when working with component properties, as opposed to global properties which I tend to keep more general.

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Now, if we open up DevTools, we can play around with the custom properties. For example, We can change --tbl-hue to a different hue value in the HSL color, set --tbl-bdrs: 0 to remove border-radius, and so on.

When working with your own components, this is the point in time you’ll discover which parameters (i.e. the custom property values) the component needs to make things look just right.

We can also use custom properties to control column alignment and width:

:where[data-component="table"] tr > *:nth-of-type(1)) {
  text-align: var(--ca1, initial);
  width: var(--cw1, initial);
  /* repeat for column 2 and 3, or use a SCSS-loop ... */
}

In DevTools, select the table and add these to the element.styles selector:

element.style {
  --ca2: center; /* Align second column center */
  --ca3: right; /* Align third column right */
}

Now, let’s create our specific component styles, using a regular class, .c-tbl (which stands for “component-table” in BEM parlance). Let’s toss that class in the table markup.

Now, let’s change the --tbl-hue value in the CSS just to see how this works before we start messing around with all of the property values:

.c-tbl {
  --tbl-hue: 330;
}

Notice, that we only need to update properties rather than writing entirely new CSS! Changing one little property updates the table’s color — no new classes or overriding properties lower in the cascade.

Notice how the border colors change as well. That’s because all the colors in the table inherit from the --tbl-hue variable

We can write a more complex selector, but still update a single property, to get something like zebra-striping:

.c-tbl tr:nth-child(even) td {
  --tbl-td-bgc: hsl(var(--tbl-hue), var(--tbl-sat), 95%);
}

And remember: It doesn’t matter where you load the class. Because our generic styles are using :where, the specificity is wiped out, and any custom styles for a specific variation will be applied no matter where they are used. That’s the beauty of using :where to take control of the cascade!

And best of all, we can create all kinds of table components from the generic styles with a few lines of CSS.

Purple table with zebra-striped columns

Light table with a “noinlineborder” parameter… which we’ll cover next

Adding parameters with another data-attribute

So far, so good! The generic table component is very simple. But what if it requires something more akin to real parameters? Perhaps for things like:

zebra-striped rows and columns
a sticky header and sticky column
hover-state options, such as hover row, hover cell, hover column

We could simply add BEM-style modifier classes, but we can actually accomplish it more efficiently by adding another data-attribute to the mix. Perhaps a data-param that holds the parameters like this:

Then, in our CSS, we can use an attribute-selector to match a whole word in a list of parameters. For example, zebra-striped rows:

[data-component="table"][data-param~="zebrarow"] tr:nth-child(even) td {
  --tbl-td-bgc: var(--tbl-zebra-bgc);
}

Or zebra-striping columns:

[data-component="table"][data-param~="zebracol"] td:nth-of-type(odd) {
  --tbl-td-bgc: var(--tbl-zebra-bgc);
}

Let’s go nuts and make both the table header and the first column sticky:


[data-component="table"][data-param~="stickycol"] thead tr th:first-child,[data-component="table"][data-param~="stickycol"] tbody tr td:first-child {
  --tbl-td-bgc: var(--tbl-zebra-bgc);
  inset-inline-start: 0;
  position: sticky;
}
[data-component="table"][data-param~="stickyrow"] thead th {
  inset-block-start: -1px;
  position: sticky;
}

Here’s a demo that allows you to change one parameter at a time:

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The default light theme in the demo is this:

.c-tbl--light {
  --tbl-bdrs: 0;
  --tbl-sat: 15%;
  --tbl-th-bgc: #eee;
  --tbl-th-bdc: #eee;
  --tbl-th-c: #555;
  --tbl-th-tt: normal;
}

…where data-param is set to noinlineborder which corresponds to these styles:

[data-param~="noinlineborder"] thead tr > th {
  border-block-start-width: 0;
  border-inline-end-width: 0;
  border-block-end-width: var(--tbl-bdw);
  border-inline-start-width: 0;
}

I know my data-attribute way of styling and configuring generic components is very opinionated. That’s just how I roll, so please feel free to stick with whatever method you’re most comfortable working with, whether it’s a BEM modifier class or something else.

The bottom line is this: embrace :where and :is and the cascade-controlling powers they provide. And, if possible, construct the CSS in such a way that you wind up writing as little new CSS as possible when creating new component variations!

Cascade Layers

The last cascade-busting tool I want to look at is “Cascade Layers.” At the time of this writing, it’s an experimental feature defined in the CSS Cascading and Inheritance Level 5 specification that you can access in Safari or Chrome by enabling the #enable-cascade-layers flag.

Bramus Van Damme sums up the concept nicely:

The true power of Cascade Layers comes from its unique position in the Cascade: before Selector Specificity and Order Of Appearance. Because of that we don’t need to worry about the Selector Specificity of the CSS that is used in other Layers, nor about the order in which we load CSS into these Layers — something that will come in very handy for larger teams or when loading in third-party CSS.

Perhaps even nicer is his illustration showing where Cascade Layers fall in the cascade:

Credit: Bramus Van Damme

At the beginning of this article, I mentioned ITCSS — a way of taming the cascade by specifying the load-order of generic styles, components etc. Cascade Layers allow us to inject a stylesheet at a given location. So a simplified version of this structure in Cascade Layers looks like this:

@layer generic, components;

With this single line, we’ve decided the order of our layers. First come the generic styles, followed by the component-specific ones.

Let’s pretend that we’re loading our generic styles somewhere much later than our component styles:

@layer components {
  body {
    background-color: lightseagreen;
  }
}

/* MUCH, much later... */

@layer generic { 
  body {
    background-color: tomato;
  }
}

The background-color will be lightseagreen because our component styles layer is set after the generic styles layer. So, the styles in the components layer “win” even if they are written before the generic layer styles.

Again, just another tool for controlling how the CSS cascade applies styles, allowing us more flexibility to organize things logically rather than wrestling with specificity.

Now you’re in control!

The whole point here is that the CSS cascade is becoming a lot easier to wrangle, thanks to new features. We saw how the :where and :is pseudo-selectors allows us to control specificity, either by stripping out the specificity of an entire ruleset or taking on the specificity of the most specific argument, respectively. Then we used CSS Custom Properties to override styles without writing a new class to override another. From there, we took a slight detour down data-attribute lane to help us add more flexibility to create component variations merely by adding arguments to the HTML. And, finally, we poked at Cascade Layers which should prove handy for specifying the loading order or styles using @layer.

If you leave with only one takeaway from this article, I hope it’s that the CSS cascade is no longer the enemy it’s often made to be. We are gaining the tools to stop fighting it and start leaning into even more.

Header photo by Stephen Leonardi on Unsplash

Don’t Fight the Cascade, Control It! originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

Cascade Layers

Chris Coyier — Tue, 21 Sep 2021 19:23:05 +0000

There is a new thing coming in CSS: @layer.

This comes from Miriam Suzanne, who is really on a tear with influencing important new CSS stuff. I’ve been hearing about all this, but then all a sudden it just dropped in experimental browsers.

Leave it to Bramus to really dig into all this with a fantastic post on it all:

With Cascade Layers coming, we developers will have more tools available to control the Cascade. The true power of Cascade Layers comes from its unique position in the Cascade: before Selector Specificity and Order Of Appearance. Because of that we don’t need to worry about the Selector Specificity of the CSS that is used in other Layers, nor about the order in which we load CSS into these Layers — something that will come in very handy for larger teams or when loading in third-party CSS.
Bramus Van Damme, “The Future of CSS: Cascade Layers (CSS @layer)“

Emphasis mine.

That’s the rub here: this is a new thing that affects which selectors win. It’s going to require some re-wiring of our CSS brains, because layers is this entirely new (and powerful) part of determining what styles actually get applied.

I say powerful because a “higher” layer can literally beat a traditionally stronger selector even with a weaker selector in the layer.

/* First layer */
@layer base-layer {
  body#foo {
    background: tan;
  }
}
/* Higher layer, so this wins, despite selector strength */
@layer theme-layer {
  body.foo {
    background: #eee;
  }
}

/* Careful! Unlayered styles are more powerful than layers, even if the selector is weaker */
body {
  background: red;
}

Because that CSS at the bottom isn’t in a layer at all, it wins, even with the weaker selector

And you aren’t limited to one layer. You get to define them and use them however you want.

@layer reset;     /* Create 1st layer named “reset” */
@layer base;      /* Create 2nd layer named “base” */
@layer theme;     /* Create 3rd layer named “theme” */
@layer utilities; /* Create 4th layer named “utilities” */
/* Or, @layer reset, base, theme, utilities; */


@layer reset { /* Append to layer named “reset” */
  /* ... */
}

@layer theme { /* Append to layer named “theme” */
  /* ... */
}

@layer base { /* Append to layer named “base” */
  /* ... */
}

@layer theme { /* Append to layer named “theme” */
  /* ... */
}

Mind-blowing, really.

How are we going to use this?

I wonder if a common pattern might turn into…

Layer everything, so priority levels are really clear. Maybe allow unlayered CSS for super powerful overrides only, but ideally even do that as a high-level layer.
Resets as the lowest layer.
Third-party stuff as the middle layer(s).
Anything team-authored as the highest layer.

You won’t have to worry about leaving space in between (like you might with z-index) because you can adjust it without needing to attach numbers at any time.

Time shall tell.

Debugging

I hope DevTools expresses layers really clearly because there is going to be some serious head-scratching for a while when we see weaker-looking selectors winning because of layer placement.

Browser Support

Looks like caniuse is on the ball here!

This browser support data is from Caniuse, which has more detail. A number indicates that browser supports the feature at that version and up.

Desktop

Chrome	Firefox	IE	Edge	Safari
99	97	No	99	15.4

Mobile / Tablet

Android Chrome	Android Firefox	Android	iOS Safari
108	107	108	15.4

Updates

This stuff is super new (at the time of writing), so volatility is to be expected, I suppose. Looks like on October 6th, 2021 it was decided that unlayered styles are actually the strongest styles, not the weakest. I’ve attempted to update the article to show that.

Cascade Layers originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

Should DevTools teach the CSS cascade?

Chris Coyier — Fri, 21 May 2021 23:22:54 +0000

Stefan Judis, two days before I mouthed off about using (X, X, X, X) for talking about specificity, has a great blog post not only using that format, but advocating that browser DevTools should show us that value by selectors.

I think that the above additions could help to educate developers about CSS tremendously. The only downside I can think of is that additional information might overwhelm developers, but I would take that risk in favor of more people learning CSS properly.

I’d be for it. The ~~crossed-off~~ UI for the “losing” selectors is attempting to teach this, but without actually teaching it. I wouldn’t be that worried about the information being overwhelming. I think if they are considerate about the design, it can be done tastefully. DevTools is a very information-dense place anyway.

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Should DevTools teach the CSS cascade? originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

Computed Values: More Than Meets the Eye

Jemimah Omodior — Wed, 05 Aug 2020 14:37:35 +0000

Browser DevTools are indispensable for us front end developers. In this article, we’ll take a look at the Computed tab, a small corner of the DevTools panel that shows us big things, like how relative CSS values are resolved. We’ll also see how inheritance fits into the browser’s style computation process.

The “Computed” tab is generally located in the right panel of the DevTools interface, like it is shown here in Chrome.

The content in the Computed tab is important because it shows us the values that the browser is actually using on the rendered website. If an element isn’t styled how you think it should be, looking at its computed values can help you understand why.

If you’re more accustomed to using the Styles tab (called Rules in Firefox), you may wonder how it differs from the Computed tab. I mean, they both show styles that apply to an element. The answer? The Computed tab displays an alphabetized list of resolved styles that include what is declared in your stylesheet, those derived from inheritance, and the browser’s defaults.

The “Computed” tab takes a selected element (1) and displays a list of CSS properties (2) that have been rendered, allowing each one to be expanded (3) to reveal the cascade of inherited values alongside the actual computed value (4) that is currently in use.

The Styles tab, on the other hand, displays the exact rulesets of a selected element exactly as they were written. So while the Styles tab might show you something like .subhead {font-size: 75%}, the Computed tab will show you the actual font size, or what 70% currently resolves to. For example, the actual font size for the rendered text as shown above is 13.2px.

The “Styles” tab takes a selected element (1) and displays the ruleset (2) that is explicitly declared in the stylesheet, followed by other related rulesets that are included in the cascade (3), including those from other stylesheets (4). Notice how overridden values are crossed out, indicating that another property takes precedence.

Next, let’s briefly review the concepts of inheritance and the cascade, two things that are a huge part of how the computed values in the Computed tab are arrived at.

Crash course on inheritance and the cascade

CSS stands for Cascading Style Sheets, and that first word cascading is incredibly important to understand – the way that the cascade behaves is key to understanding CSS.
MDN

The cascade is notable because it’s the “C” in CSS. It’s the mechanism used for resolving conflicts that exist between the different sources of style declarations for a document.

For example, imagine a stylesheet that defines the width of a div twice:

div {
  width: 65vw;
}
 
/* Somewhere, further down */
div {
  width: 85vw;
}

In this specific example, the second width wins out since it is declared last. The first width could still win with !important but that’s technically breaking the cascade by brute force. The point here is that the cascade algorithm determines what styles apply to each element and prioritizes them in a predetermined order to settle on a value.

The cascade is applied for properties that are set explicitly, whether by the browser, the web developer, or the user. Inheritance comes into the picture when the output of the cascade is empty. When this happens, the computed value of a property on an element’s parent is pulled in as its own value for that property. For example, if you specify a color for an element, all child elements will inherit that color if you don’t specify theirs.

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There are four key property values related to inheritance that we should get acquainted with before we plow ahead. We’ll be using these throughout the article.

`initial`

In an HTML document where the highest level of the DOM tree is the element, when we use the initial keyword on an element like this…

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…the text color for that element is black, even though the body element is set to green. There’s the matter of the div selector having a higher specificity, however we’re interested in why initial translated to black.

In plain terms, this keyword sets the default value of a property as specified in its definition table (in the CSS specs). In this case, black happens to be the browser’s implementation of the initial color value.

I mention near the end of the article that you can learn whether or not a property is inherited by default by checking out its page on MDN. Well, you can also find the initial value for any property this way.

`inherit`

For non-inherited properties, this keyword forces inheritance. In the following example, the element has a solid red border. The border property isn’t inherited by default, but we can tell our div to inherit the same red border declared on the element by using the inherit keyword on its border property:

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`unset`

unset will resolve to an inherited value if a property is inherited. Otherwise, the initial value is used. This basically means unset resets a property based on whether it is inherited or not. Here’s a demo that toggles unset to show its effect on elements with different levels of specificity.

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`revert`

If no CSS properties are set on an element, then does it get any styles at all? You bet. It uses the browser’s default styles.

For example, the initial value for the display property for span elements is inline, but we can specify it as block in our stylesheet. Use the button in the following demo to toggle revert on both the span element’s display and color properties:

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The span properly reverts to an inline element, but wait! Did you notice that the color of the span goes to a green color instead of the browser’s default black value? That’s because revert allows for inheritance. It will go as far back as the browser’s default to set the color, but since we’ve explicitly set a green color on the element, that’s what is inherited.

Finding computed values in DevTools

This is where we start talking about the computed values in DevTools. Just as with the default values of properties, the computed value of a CSS property is determined by that property’s definition table in the CSS specifications. Here’s what that looks like for the height property.

Say we use relative lengths in our CSS, like one of 10em or 70% or 5vw. Since these are “relative” to something – font-size or the viewport – they’ll need to get resolved to a pixel-absolute value. For example, an element with a 10% width may compute to 100px if the viewport is 1000px wide, but some other number altogether when the viewport width changes.

A button (1) is the current selected element in DevTools (2). The declared width of the button is 100% (3), which computes to 392px (4) when the viewport is in this condition.

These values are calculated whenever the DOM is modified in a process called computed styles calculation. This is what lets the browser know what styles to apply to each page element.

Style calculations happen in multiple steps involving several values. These are documented in the CSS Cascading and Inheritance Level 4 specification and they all impact the final value we see in the Computed tab. Let’s take a look at those next.

Values and how they’re processed

The values defined for the style calculation process include the declared value, the specified value, the cascaded value, the computed value, the used value, and the actual value. Who knew there were so many, right?

Declared values

A declared value is any property declaration applies to an element. A browser identifies these declarations based on a few criteria, including:

the declaration is in a stylesheet that applies to the current document
there was a matching selector in a style declaration
the style declaration contains valid syntax (i.e, valid property name and value)

Take the following HTML:


  It's not denial. I'm just selective about the reality I accept.

Here are declared values that apply to the font-size of the text:

main {
  font-size: 1.2em; /* this would apply if the paragraph element wasn't targeted specifically, and even then, as an inherited value, not "declared value" */
}
 
main > p {
  font-size: 1.5em; /* declared value */
}

Cascaded values

The list of all declared values that apply to an element are prioritized based things like these to return a single value:

origin of the declaration (is it from the browser, developer, or another source?)
whether or not the declaration is marked ‘!important’
how specific a rule is (e.g, span {} vs section span {})
order of appearance (e.g, if multiple declarations apply, the last one will be used)

In other words, the cascaded value is the “winning” declaration. And if the cascade does not result in a winning declared value, well, then there is no cascaded value.

main > p  {
  font-size: 1.2em;
}
 
main > .product-description { /* the same paragraph targeted in the previous rule */
  font-size: 1.2em; /* cascaded value based on both specificity and document order, ignoring all other considerations such as origin */
}

Specified values

As mentioned earlier, it is possible for the output of the cascade to be empty. However, a value still needs to be found by other means.

Now, let’s say we didn’t declare a value for a specific property on an element, but did for the parent. That’s something we often do intentionally because there’s no need to set the same value in multiple places. In this case, the inherited value for the parent is used. This is called the specified value.

In many cases, the cascaded value is also the specified value. However, it can also be an inherited value if there is no cascaded value and the property concerned is inherited, whether by default or using the inherit keyword. If the property is not inherited, then the specified value is the property’s initial value, which, as mentioned earlier, can also be set explicitly using the initial keyword.

In summary, the specified value is the value we intend to use on an element, with or without explicitly declaring it on that element. This is a little murky because the browser’s default can also become the specified value if nothing is declared in the stylesheet.

/* Browser default = 16px */
 
main > p {
  /* no declared value for font-size for the paragraph element and all its ancestors */
}

Computed values

Earlier, we discussed, briefly, how relative values needed to be resolved to their pixel-absolute equivalent. This process, as already noted, is pre-determined. For example, property definition tables have a “Computed value” field that detail how specified values, in general, are resolved.

The specifications section of the MDN docs for the color property.

In the following example, we’re working with the em, a relative unit. Here, the final value used when rendering the element to which the property applies is not a fixed number as seen in our declared value, but something that needs to be calculated based on a few factors.

main {
  font-size: 1.2em;
}
 
main > p {
  font-size: 1.5em; /* declared value */
}

The font-size of the paragraph element is set to 1.5em, which is relative to the font-size value of the main element, 1.2em. If main is a direct child of the body element – and no additional font-size declarations are made above that, such as by using the :root selector – we can assume that the calculation for the paragraph’s font-size will follow this approximate course:

Browser_Default_FontSize = 16px;
Calculated_FontSize_For_Main = 1.2 * Browser_Default_FontSize; // 19.2px
Calculated_FontSize_For_Paragraph = 1.5 * Calculated_FontSize_For_Main; // 28.8px

That 28.8px is the computed value. Here’s a demo:

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Open up DevTools and check out the computed font sizes in the Computed tab.

The declared font-size for the main element is 1.2em, which computes to 19.2px.

The declared font-size for the paragraph element is 1.5em, which computes to 28.8px.

Let’s say we’re using rem units instead:

html {
  font-size: 1.2em;
}
 
main {
  font-size: 1.5rem;
}
 
div {
  font-size: 1.7rem;
}

The computed value of a rem unit is based on the font-size of the root HTML element, so that means that the calculation changes a little bit. In this specific case, we’re using a relative unit on the HTML element as well, so the browser’s default font-size value is used to calculate the base font-size we’ll use to resolve all our rem values.

Browser_Default_FontSize = 16px
Root_FontSize = 1.2 * Browser_Default_FontSize; // 19.2px
Calculated_FontSize_For_Main = 1.5 * Root_FontSize; // 28.8px
Calculated_FontSize_For_Div = 1.7 * Root_FontSize; // 32.64px

Open up DevTools again for this demo:

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The value, 16px, for Browser_Default_FontSize is commonly used by browsers, but this is subject to variation. To see your current default, select the element in DevTools and check out the font-size that is shown for it. Note that if a value was set for the root element explicitly, just as in our example, you may have to toggle it off in the Rules tab. Next, toggle on the “Show all” or “Browser styles” (Firefox) checkbox in the Computed tab to see the default.

During inheritance, computed values are passed down to child elements from their parents. The computation process for this takes into account the four inheritance-controlling keywords we looked at earlier. In general, relative values become absolute (i.e. 1rem becomes 16px). This is also where relative URLs become absolute paths, and keywords such as bolder (value for the font-weight property) get resolved. You can see some more examples of this in action in the docs.

Used values

The used value is the final result after all calculations are done on the computed value. Here, all relative values are turned absolute. This used value is what will be applied (tentatively) in page layout. You might wonder why any further calculations have to happen. Wasn’t it all taken care of at the previous stage when specified values were processed to computed values?

Here’s the thing: some relative values will only be resolved to pixel-absolutes at this point. For example, a percentage-specified width might need page layout to get resolved. However, in many cases, the computed value winds up also being the used value.

Note that there are cases where a used value may not exist. According to the CSS Cascading and Inheritance Level 4 specification:

…if a property does not apply to an element, it has no used value; so, for example, the flex property has no used value on elements that aren’t flex items.

Actual values

Sometimes, a browser is unable to apply the used value straightaway and needs to make adjustments. This adjusted value is called the actual value. Think of instances where a font size needs to be tweaked based on available fonts, or when the browser can only use integer values during rendering and need to approximate non-integer values.

Inheritance in browser style computations

To recap, inheritance controls what value is applied to an element for a property that isn’t set explicitly. For inherited properties, this value is taken from whatever is computed on the parent element, and for non-inherited properties, the initial value for that property is set (the used value when the keyword initial is specified).

We talked about the existence of a “computed value” earlier, but we really need to clarify something. We discussed computed values in the sense of one type of value that takes part in the style resolution process, but “computed value” is also a general term for values computed by the browser for page styling. You’ll typically understand which kind we mean by the surrounding context.

Only computed values are accessible to an inherited property. A pixel-absolute value such as 477px, a number such as 3, or a value such as left (e.g. text-align: left) is ready for the inheritance process. A percentage value like 85% is not. When we specify relative values for properties, a final (i.e. “used”) value has to be calculated. Percentage values or other relative values will be multiplied by a reference size (font-size, for instance) or value (e.g. the width of your device viewport). So, the final value for a property can be just what was declared or it might need further processing to be used.

You may or may not have already noticed, but the values shown in the Computed tab of the browser will not necessarily be the computed values we discussed earlier (as in computed vs. specified or used values). Rather, the values shown are the same as returned by the getComputedStyle() function. This function returns a value which, depending on the property, will either be the computed value or the used value.

Now, let’s see some examples.

Color inheritance

main {
  color: blue;
}

/* The color will inherit anyway, but we can be explicit too: */
main > p {
  color: inherit;
}

The value computed for the color property on the main element will be blue. As color is inherited by default, we really didn’t need color: inherit for the paragraph child element because it would wind up being blue anyway. But it helps illustrate the point.

Color values undergo their own resolution process to become used values.

Font size inheritance

main {
  font-size: 1.2em;
}

main > p {
  /* No styles specified */
}

As we saw earlier in the section on values and how they are processed, our relative value for font-size will compute to an absolute value and then be inherited by the paragraph element, even if we don’t explicitly declare it (again, font-size is inherited by default). If we had previously set styles via a global paragraph element selector, then the paragraph may gain some extra styles by virtue of the cascade. Any property values that may be inherited will be, and some properties for which the cascade and inheritance didn’t produce a value will be set to their initial value.

Percentage-specified font size inheritance

body {
  font-size: 18px;
}

main {
  font-size: 80%;
}

main > p {
  /* No styles specified */
}

Similar to the previous example, the

element’s font-size will be absolutized in preparation for inheritance and the paragraph will inherit a font-size that is 80% of the body’s 18px value, or 14.4px.

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Forced inheritance and post-layout computation

Computed values generally resolve the specified value as much as possible without layout, but as mentioned earlier, some values can only be resolved post-layout, such as percentage-specified width values. Although width isn’t an inherited property, we can force inheritance for the purpose of illustrating pre-layout and post-layout style resolution.

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This is a contrived example but what we’re doing is taking an element out of the page layout by setting its display property to none. We have two divs in our markup that inherit a width, 50%, from their parent element

. In the Computed tab in DevTools, the computed width for the first div is absolute, having been resolved to a pixel value (243.75px for me). On the other hand, the width of the second div that was taken out of the layout using display: none is still 50%.

We’ll imagine that the specified and computed value for the parent

element is 50% (pre-layout) and the used value is as shown under the Computed tab – that’s 487.5px for me, post-layout. This value is halved for inheritance by the child divs (50% of the containing block).

These values have to be computed whenever the width of the browser’s viewport changes. So, percentage-specified values become percentage-computed values, which become pixel-used values.

Properties that inherit by default

How do you know if a property inherits by default or not? For each CSS property in the MDN docs, there is a specifications section that provides some extra details that include whether or not the property is inherited. Here’s what that looks like for the color property:

The specifications section of the MDN docs for the color property.

Which properties are inherited by default and which aren’t is largely down to common sense.
MDN

Another reference option is the properties section of the W3C specs. Still another is this StackOverflow thread which may not be exhaustive at the time of writing.

Here are some examples of properties that inherit by default:

Examples of properties that do not (but which you can force to inherit with the inherit keyword):

Hopefully this gives you a solid idea of how browsers compute styles and how to reference them in DevTools. As you can see, there’s a lot that goes into a value behind the scenes. Having that context goes a long way in helping you troubleshoot your work as well as furthering your general understanding of the wonderful language we know as CSS.

Using Custom Property “Stacks” to Tame the Cascade

Miriam Suzanne — Mon, 22 Jun 2020 14:47:49 +0000

Since the inception of CSS in 1994, the cascade and inheritance have defined how we design on the web. Both are powerful features but, as authors, we’ve had very little control over how they interact. Selector specificity and source order provide some minimal “layering” control, without a lot of nuance — and inheritance requires an unbroken lineage. Now, CSS Custom Properties allow us to manage and control both cascade and inheritance in new ways.

I want to show you how I’ve used Custom Property “stacks” to solve some of the common issues people face in the cascade: from scoped component styles, to more explicit layering of intents.

A quick intro to Custom Properties

The same way browsers have defined new properties using a vendor prefix like -webkit- or -moz-, we can define our own Custom Properties with an “empty” -- prefix. Like variables in Sass or JavaScript, we can use them to name, store, and retrieve values — but like other properties in CSS, they cascade and inherit with the DOM.

/* Define a custom property */
html {
  --brand-color: rebeccapurple;
}

In order to access those captured values, we use the var() function. It has two parts: first the name of our custom property, and then a fallback in case that property is undefined:

button {
  /* use the --brand-color if available, or fall back to deeppink */
  background: var(--brand-color, deeppink);
}

This is not a support fallback for old browsers. If a browser doesn’t understand custom properties, it will ignore the entire var() declaration. Instead, this is a built-in way of handling undefined variables, similar to a font stack defining fallback font families when one is unavailable. If we don’t provide a fallback, the default is unset.

Building variable “stacks”

This ability to define a fallback is similar to “font stacks” used on the font-family property. If the first family is unavailable, the second will be used, and so on. The var() function only accepts a single fallback, but we can nest var() functions to create custom-property fallback “stacks” of any size:

button {
  /* try Consolas, then Menlo, then Monaco, and finally monospace */
  font-family: Consolas, Menlo, Monaco, monospace;

  /* try --state, then --button-color, then --brand-color, and finally deeppink */
  background: var(--state, var(--button-color, var(--brand-color, deeppink)));
}

If that nested syntax for stacked properties looks bulky, you can use a pre-processor like Sass to make it more compact.

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That single-fallback limitation is required to support fallbacks with a comma inside them — like font stacks or layered background images:

html {
  /* The fallback value is "Helvetica, Arial, sans-serif" */
  font-family: var(--my-font, Helvetica, Arial, sans-serif);
}

Defining “scope”

CSS selectors allow us to drill down into the HTML DOM tree, and style elements anywhere on the page, or elements in a particular nested context.

/* all links */
a { color: slateblue; }

/* only links inside a section */
section a { color: rebeccapurple; }

/* only links inside an article */
article a { color: deeppink; }

That’s useful, but it doesn’t capture the reality of “modular” object-oriented or component-driven styles. We might have multiple articles and asides, nested in various configurations. We need a way to clarify which context, or scope, should take precedence when they overlap.

Proximity scopes

Let’s say we have a .light theme and a .dark theme. We can use those classes on the root element to define a page-wide default, but we can also apply them to specific components, nested in various ways:

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Each time we apply one of our color-mode classes, the background and color properties are reset, then inherited by nested headings and paragraphs. In our main context, colors inherit from the .light class, while the nested heading and paragraph inherit from the .dark class. Inheritance is based on direct lineage, so the nearest ancestor with a defined value will take precedence. We call that proximity.

Proximity matters for inheritance, but it has no impact on selectors, which rely on specificity. That becomes a problem if we want to style something inside the dark or light containers.

Here I’ve attempted to define both light and dark button variants. Light mode buttons should be rebeccapurple with white text so they stand out, and dark mode buttons should be plum with black text. We’re selecting the buttons directly based on a light and dark context, but it doesn’t work:

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Some of the buttons are in both contexts, with both .light and .dark ancestors. What we want in that case is for the closest theme to take over (inheritance proximity behavior), but what we get instead is the second selector overriding the first (cascade behavior). Since the two selectors have the same specificity, source order determines the winner.

Custom Properties and proximity

What we need here is a way to inherit these properties from the theme, but only apply them to specific children. Custom Properties make that possible! We can define values on the light and dark containers, while only using their inherited values on nested elements, like our buttons.

We’ll start by setting up the buttons to use custom properties, with a fallback “default” value, in case those properties are undefined:

button {
  background: var(--btn-color, rebeccapurple);
  color: var(--btn-contrast, white);
}

Now we can set those values based on context, and they will scope to the appropriate ancestor based on proximity and inheritance:

.dark {
  --btn-color: plum;
  --btn-contrast: black;
}

.light {
  --btn-color: rebeccapurple;
  --btn-contrast: white;
}

As an added bonus, we’re using less code overall, and one unified button definition:

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I think of this as creating an API of available parameters for the button component. Sara Soueidan and Lea Verou have both covered this well in recent articles.

Component ownership

Sometimes proximity isn’t enough to define scope. When JavaScript frameworks generate “scoped styles” they are establishing specific object-element ownership. A “tab layout” component owns the tabs themselves, but not the content behind each tab. This is also what the BEM convention attempts to capture in complex .block__element class names.

Nicole Sullivan coined the term “donut scope” to talk about this problem back in 2011. While I’m sure she has more recent thoughts on the issue, the fundamental problem hasn’t changed. Selectors and specificity are great for describing how we build detailed styles over top of broad patterns, but they don’t convey a clear sense of ownership.

We can use custom property stacks to help solve this problem. We’ll start by creating “global” properties on the element that are for our default colors:

html {
  --background--global: white;
  --color--global: black;
  --btn-color--global: rebeccapurple;
  --btn-contrast--global: white;
}

That default global theme is now available anywhere we want to refer to it. We’ll do that with a data-theme attribute that applies our foreground and background colors. We want the global values to provide a default fallback, but we also want the option to override with a specific theme. That’s where “stacks” come in:

[data-theme] {
  /* If there's no component value, use the global value */
  background: var(--background--component, var(--background--global));
  color: var(--color--component, var(--color--global));
}

Now we can define an inverted component by setting the *--component properties as a reverse of the global properties:

[data-theme='invert'] {
  --background--component: var(--color--global);
  --color--component: var(--background--global);
}

But we don’t want those settings to inherit beyond the donut of ownership, so we reset those values to initial (undefined) on every theme. We’ll want to do this at a lower specificity, or earlier in the source order, so it provides a default that each theme can override:

[data-theme] {
  --background--component: initial;
  --color--component: initial;
}

The initial keyword has a special meaning when used on custom properties, reverting them to a Guaranteed-Invalid state. That means rather than being passed along to set background: initial or color: initial, the custom property becomes undefined, and we fallback to the next value in our stack, the global settings.

We can do the same thing with our buttons, and then make sure to apply data-theme to each component. If no specific theme is given, each component will default to the global theme:

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Defining “origins”

The CSS cascade is a series of filtering layers used to determine what value should take precedence when multiple values are defined on the same property. We most often interact with the specificity layers, or the final layering based on source-order — but the first layer of cascade is the “origin” of a style. The origin describes where a style came from — often the browser (defaults), the user (preferences), or the author (that’s us).

By default, author styles override user preferences, which override browser defaults. That changes when anyone applies `!important` to a style, and the origins reverse: browser `!important` styles have the highest origin, then important user preferences, then our author important styles, above all the normal layers. There are a few additional origins, but we won’t go into them here.

When we create custom property “stacks,” we’re building a very similar behavior. If we wanted to represent existing origins as a stack of custom properties, it would look something like this:

.origins-as-custom-properties {
  color: var(--browser-important, var(--user-important, var(--author-important, var(--author, var(--user, var(--browser))))));
}

Those layers already exist, so there’s no reason to recreate them. But we’re doing something very similar when we layer our “global” and “component” styles above — creating a “component” origin layer that overrides our “global” layer. That same approach can be used to solve various layering issues in CSS, which can’t always be described by specificity:

Override » Component » Theme » Default
Theme » Design system or framework
State » Type » Default

Let’s look at some buttons again. We’ll need a default button style, a disabled state, and various button “types,” like danger, primary and secondary. We wan’t the disabled state to always override the type variations, but selectors don’t capture that distinction:

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But we can define a stack that provides both “type” and “state” layers in the order that we want them prioritized:

button {
  background: var(--btn-state, var(--btn-type, var(--btn-default)));
}

Now when we set both variables, the state will always take precedence:

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I’ve used this technique to create a Cascading Colors framework that allows custom theming based on layering:

Pre-defined theme attributes in the HTML
User color preferences
Light and dark modes
Global theme defaults

Mix and match

These approaches can be taken to an extreme, but most day-to-day use-cases can be handled with two or three values in a stack, often using a combination of the techniques above:

A variable stack to define the layers
Inheritance to set them based on proximity and scope
Careful application of the `initial` value to remove nested elements from a scope

We’ve been using these custom property “stacks” on our projects at OddBird. We’re still discovering as we go, but they’ve already been helpful in solving problems that were difficult using only selectors and specificity. With custom properties, we don’t have to fight the cascade or inheritance. We can capture and leverage them, as-intended, with more control over how they should apply in each instance. To me, that’s a big win for CSS — especially when developing style frameworks, tools, and systems.

Using Custom Property “Stacks” to Tame the Cascade originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

The Order of CSS Classes in HTML Doesn’t Matter

Robin Rendle — Tue, 17 Dec 2019 15:04:07 +0000

That’s right! And I can prove it, too. Let’s look at some CSS first:

.a {
  color: red;
}

.b {
  color: blue;
}

And now let’s look at some markup:

Here’s some text

The text is going to be blue because .b is defined last in the CSS, right? But what if we go about and switch the order in which those classes are called in HTML:

Here’s some text

What color do you think the text should be? Red or blue?

This certainly might sound like a silly question but it tends to trip up a lot of folks who happen to be familiar with CSS-in-JS solutions. And this week I’ve spoken to two very senior front end engineers who thought similarly as well!

But the text in the example above will always be blue no matter what order those CSS classes are in. And that’s because the markup is just reading the CSS in the order that it’s written — the cascade wins in this example.

The Order of CSS Classes in HTML Doesn’t Matter originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

The “C” in CSS: The Cascade

Thomas Yip — Tue, 13 Nov 2018 15:01:24 +0000

Following up from Geoff’s intro article on The Second “S” in CSS, let’s now move the spotlight to the “C” in CSS — what we call the Cascade. It’s where things start to get messy, and even confusing at times.

Have you ever written a CSS property and the value doesn’t seem to work? Maybe you had to turn to using !important to get it going. Or perhaps you resorted to writing the CSS inline on the element in the HTML file.


  Ack, inline!

Speaking of inline styles, have you wondered why SVG editors use them instead of a separate CSS file? That seems kinda weird, right?

Well, the cascade has a lot to do with this. Read on to find out how styling methods affect what’s being applied to your elements and how to use the cascade to your advantage because, believe me, it’s a wonderful thing when you get the hang of it.

TL;DR: Jump right to the CSS order diagram for a visual of how everything works.

The cascade cares about how and where styles are written

There are a myriad of ways you can apply CSS rules to an element. Below is an example of how stroke: red; can be applied to the same element. The examples are ordered in ascending priority, where the highest priority is at the bottom:



   












rect { stroke: red; }
.myClass { stroke: red; }
#myID { stroke: red; }

Inheritance? Embedded? External? Inline? Specificity? Important? Yeah, lots of terms being thrown around. Let’s break those down a bit because each one determines what the browser ends up using when a web page loads.

Elements can inherit styles from other elements

Both HTML and SVG elements can inherit CSS rules that are applied to other elements. We call this a parent-child relationship, where the element the CSS is applied to is the parent and the element contained inside the parent is the child.


  I'm the child because the parent is wrapped around me.

If we set the text color of the parent and do not declare a text color on the child, then the child will look up to the parent to know what color its text should be. We call that inheritance and it’s a prime example of how a style cascades down to an element it matches… or “bubbles up” the chain to the next matched style.

However, inheritance has the lowest priority among styling methods. In other words, if a child has a rule that is specific to it, then the inherited value will be ignored, even though the inherited value may have an important keyword. The following is an example:


  I'm the child because the parent is wrapped around me.

See the Pen Child ignores inline inheritance with !important by Geoff Graham (@geoffgraham) on CodePen.

SVG inline attributes

For SVG elements, we can also apply styles using inline attributes, where those have the second lowest priority in the cascade. This means the CSS rules in a stylesheet will be able to override them.

rect {
  stroke: blue;
}

See the Pen Stylesheet overrides SVG inline attributes by Geoff Graham (@geoffgraham) on CodePen.

Most SVG editors use inline attributes for portability; that is, the ability to copy some elements and paste them elsewhere without losing the attributes. Users can then use the resultant SVG and style its elements using an external stylesheet.

Stylesheets

Stylesheets are divided into two flavors: external and embedded:

Embedded styles have the same priority as external stylesheets. Therefore, if you have the same CSS rules, ordering rules applies.

See the Pen Embedded styles override stylesheet rules by Geoff Graham (@geoffgraham) on CodePen.

All stylesheets follow ordering rules, where files that are defined later, will have higher priority than those defined earlier. In this example, stylesheet-2.css will take precedence over the stylesheet-1.css file because it is defined last.

Specificity or selectors

How you select your elements will also determine which rules are applied, whereby tags (e.g.

), classes (e.g. .my-class) and IDs (e.g. #myI-id) have ascending priorities.

See the Pen Specificity by selectors by Geoff Graham (@geoffgraham) on CodePen.

In the example above, if you have a div element with both .my-class and #my-id, the border will be red because IDs have higher priority than classes and tags.

*Specificity has higher priority than ordering rules, therefore, irrespective if your rule is at the top or bottom. Specificity still has higher priority and will be applied.

Ordering

CSS rules always prioritize from left-to-right, then from top-to-bottom.

Inline styles

Inline styles have the second highest priority, just below the !important keyword. This means that inline styles are only overridden by the important keyword and nothing else. Within inline styles, normal ordering rules applies, from left-to-right and top-to-bottom.

The important keyword

Speaking of the !important keyword, it is used to override ordering, specificity and inline rules. In other words, it wields incredible powers.

Overriding inline rules

In the example above, without the important keyword, the div would have a red border because inline styling has higher priority than embedded styles. But, with the important keyword, the div border becomes orange, because the important keyword has higher priority than inline styling.

Using !important can be super useful, but should be used with caution. Chris has some thoughts on situations where it makes sense to use it.

Overriding specificity rules

Without the important keyword, this div border will be blue, because classes have higher priority than tags in specificity.

See the Pen Classes beat tags by Geoff Graham (@geoffgraham) on CodePen.

But! Adding the important keyword to the tag rules tells the element to ignore the cascade and take precedence over the class rules.

See the Pen !important ignores the cascade by Geoff Graham (@geoffgraham) on CodePen.

Overriding ordering rules

OK, so we’ve already talked about how the order of rules affects specificity: bottom beats top and right beats left. The surefire way to override that is to put !important into use once again.

In this example, the div will take the red border, even though the blue border is the bottom rule. You can thank !important for that handiwork.

See the Pen Important wins over ordering by Geoff Graham (@geoffgraham) on CodePen.

Visualizing the cascade

Who knew there was so much meaning in the “C” of CSS? We covered a ton of ground here and hopefully it helps clarify the way styles are affected and applied by how we write them. The cascade is a powerful feature. There are opinions galore about how to use it properly, but you can see the various ways properties are passed and inherited by elements.

More of a visual learner? Here’s a chart that pulls it all together.

Download chart

The “C” in CSS: The Cascade originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

CSS Basics: The Second “S” in CSS

Geoff Graham — Wed, 14 Feb 2018 14:14:24 +0000

CSS is an abbreviation for Cascading Style Sheets.

While most of the discussion about CSS on the web (or even here on CSS-Tricks) is centered around writing styles and how the cascade affects them, what we don’t talk a whole lot about is the sheet part of the language. So let’s give that lonely second “S” a little bit of the spotlight and understand what we mean when we say CSS is a style sheet.

The Sheet Contains the Styles

The cascade describes how styles interact with one another. The styles make up the actual code. Then there’s the sheet that contains that code. Like a sheet of paper that we write on, the “sheet” of CSS is the digital file where styles are coded.

If we were to illustrate this, the relationship between the three sort of forms a cascade:

The sheet holds the styles.

There can be multiple sheets all continuing multiple styles all associated with one HTML document. The combination of those and the processes of figuring out what styles take precedence to style what elements is called the cascade (That first “C” in CSS).

The Sheet is a Digital File

The sheet is such a special thing that it’s been given its own file extension: .css. You have the power to create these files on your own. Creating a CSS file can be done in any text editor. They are literally text files. Not “rich text” documents or Word documents, but plain ol’ text.

If you’re on Mac, then you can fire up TextEdit to start writing CSS. Just make sure it’s in “Plain Text” mode.

If you’re on Windows, the default Notepad app is the equivalent. Heck, you can type styles in just about any plain text editor to write CSS, even if that’s not what it says it was designed to do.

Whatever tool you use, the key is to save your document as a .css file. This can usually be done by simply add that to your file name when saving. Here’s how that looks in TextEdit:

Seriously, the choice of which text editor to use for writing CSS is totally up to you. There are many, many to choose from, but here are a few popular ones:

You might reach for one of those because they’ll do handy things for you like syntax highlight the code (colorize different parts to help it be easier to understand what is what).

Hey look I made some files completely from scratch with my text editor:

Those files are 100% valid in any web browser, new or old. We’ve quite literally just made a website.

The Sheet is Linked Up to the HTML

We do need to connect the HTML and CSS though. As in make sure the styles we wrote in our sheet get loaded onto the web page.

A webpage without CSS is pretty barebones:

See the Pen Style-less Webpage by Geoff Graham (@geoffgraham) on CodePen.

Once we link up the CSS file, voila!

See the Pen Webpage With Styles by Geoff Graham (@geoffgraham) on CodePen.

How did that happen? if you look at the top of any webpage, there’s going to be a tag that contains information about the HTML document:

Even though the code inside the might look odd, there is typically one line (or more, if we’re using multiple stylesheets) that references the sheet. It looks something like this:

This line tells the web browser as it reads this HTML file:

I’d like to link up a style sheet
Here’s where it is located

You can name the sheet whatever you want:

styles.css
global.css
seriously-whatever-you-want.css

The important thing is to give the correct location of the CSS file, whether that’s on your web server, a CDN or some other server altogether.

Here are a few examples:

The Sheet is Not Required for HTML

You saw the example of a barebones web page above. No web page is required to use a stylesheet.

Also, we can technically write CSS directly in the HTML using the HTML style attribute. This is called inline styling and it goes a little something like this if you imagine you’re looking at the code of an HTML file:

A Headline
Some paragraph content.

While that’s possible, there are three serious strikes against writing styles this way:

If you decide to use a stylesheet later, it is extremely difficult to override inline styles with the styles in the HTML. Inline styles take priority over styles in a sheet.
Maintaining all of those styles is tough if you need to make a “quick” change and it makes the HTML hard to read.
There’s something weird about saying we’re writing CSS inline when there really is no cascade or sheet. All we’re really writing are styles.

There is a second way to write CSS in the HTML and that’s directly in the in a

That does indeed make the HTML easier to read, already making it better than inline styling. Still, it’s hard to manage all styles this way because it has to be managed on each and every webpage of a site, meaning one “quick” change might have to be done several times, depending on how many pages we’re dealing with.

An external sheet that can be called once in the is usually your best bet.

The Sheet is Important

I hope that you’re starting to see the importance of the sheet by this point. It’s a core part of writing CSS. Without it, styles would be difficult to manage, HTML would get cluttered, and the cascade would be nonexistent in at least one case.

The sheet is the core component of CSS. Sure, it often appears to play second fiddle to the first “S” but perhaps that’s because we all have an quiet understanding of its importance.

Leveling Up

Now that you’re equipped with information about stylesheets, here are more resources you jump into to get a deeper understanding for how CSS behaves:

Specifics on Specificity – The cascade is a confusing concept and this article breaks down the concept of specificity, which is a method for how to manage it.
The latest ways to deal with the cascade, inheritance and specificity – That’s a lot of words, but the this article provides pro tips on how to manage the cascade, including some ideas that may be possible in the future.
Override Inline Styles with CSS – This is an oldie, but goodie. While the technique is probably not best practice today, it’s a good illustration of how to override those inline styles we mentioned earlier.
When Using !important is The Right Choice – This article is a perfect call-and-response to the previous article about why that method may not be best practice.

CSS Basics: The Second “S” in CSS originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

The latest ways to deal with the cascade, inheritance and specificity

Ollie Williams — Wed, 10 Jan 2018 14:24:16 +0000

The cascade is such an intrinsic part of CSS that they put it right there in the name. If you’ve ever needed to use !important to affect specificity in the cascade, you’ll know that it can be a tricky thing to deal with. In the early days of CSS, it was common to see highly specific selectors like this:

#sidebar ul li {}

We’re all much better at managing specificity nowadays. It’s a widely accepted best practice to keep specificity low and flat—to shun ID selectors, to make liberal use of classes, and to avoid unnecessary nesting. But there are still plenty of situations where a more specific selector will be useful. With the introduction of a newly proposed pseudo-class, more support of the shadow DOM, and the use of the all property, we will soon be able to handle inheritance and specificity in new and exciting ways.

The `:is()` Pseudo-Class

Lea Verou recently proposed this new pseudo-class specifically designed to control specificity. It’s already made its way to the CSS Level 4 Selectors spec. Lea has a write up of why it’s useful and there’s some coverage of it in the CSS-Tricks almanac.

Let’s take :not as an example. The specificity of :not is equal to the specificity of its argument. This makes using :not rather painful. Take the following as an example:

CodePen Embed Fallback

We might expect that the .red class would have higher specificity because it is lower in the cascade. However, for any styles to override div:not(.foobar) they would need to at least match the specificity of a combined element selector (div) and class selector (.foobar). Another approach would be div.red, but there is a better way. This is where :is can help.

div:is(:not(.foobar)) {
  background-color: black;
}

The :not selector no longer adds any specificity, so the total specificity of the above selector is simply that of one element selector (div). The .red class would now be able to override it in the cascade. Once implemented, specificity hacks will be a thing of the past.

Shadow DOM

Today, many people are using classes in HTML like this:

When using shadow DOM, rather than following a verbose naming convention, we’ll be able to omit classes altogether. Styles defined within the shadow DOM are scoped to apply only within the component. Styling can be achieved with simple element selectors without worrying about whether the selectors will interfere with elements elsewhere on the page.

CodePen Embed Fallback

It’s liberating to write such easy CSS. No more effort spent naming things. Shadow DOM looks like it is finally making its way to full browser support. It’s likely to make it into the next release of Firefox while Edge have implementation as a high priority.

This browser support data is from Caniuse, which has more detail. A number indicates that browser supports the feature at that version and up.

Desktop

Chrome	Firefox	IE	Edge	Safari
53	63	No	79	10

Mobile / Tablet

Android Chrome	Android Firefox	Android	iOS Safari
108	107	108	11.0-11.2

The `all` Property

The all property is a way of setting all CSS properties at once—everything from align-content to z-index. What values does it accept? I can’t think of any use case when I’d want all properties to inherit, but that’s an option. Then there’s initial which is more like applying a CSS reset where all the styles are gone. No padding. No margin. The initial value is set per property, regardless of the element it is applied to. The initial value of display is inline, even if you apply it to a div. The font-style of an em tag is normal, as is the font-weight of a strong tag. Link text will be black. You get the idea. (You can find the initial value of any CSS property on MDN.) This does perhaps limit its usefulness, going further than we might like by removing all styles, regardless of context.

CodePen Embed Fallback

Sadly, the most useful value for all is also the least widely implemented: revert. It can remove the styles that you as a developer have applied, while leaving the default user-agent styles intact. We’ve all seen a page of HTML without a stylesheet—black Times New Roman on a white (transparent) background with blue underlined links. If you really want to avoid inheritance, then all: revert has you covered. All divs will be display: block and spans will be inline. All em tags will be italic and strong tags will be bold. Links will be blue and underlined.

CodePen Embed Fallback

This browser support data is from Caniuse, which has more detail. A number indicates that browser supports the feature at that version and up.

Desktop

Chrome	Firefox	IE	Edge	Safari
84	67	No	84	9.1

Mobile / Tablet

Android Chrome	Android Firefox	Android	iOS Safari
108	107	108	9.3

The future?

CSS-in-JS is a cry for help. We at @csswg should pay attention to this and address the issues before it gets worse.https://t.co/lWQ4ct61ir
— Lea Verou (@LeaVerou) May 24, 2017

The miscellany of rival unstandardized methods for writing CSS-in-JS was an attempt to sidestep these same issues. That approach has gained popularity over the last several years. Some of its proponents have deemed inheritance, the cascade and specificity as fundamentally flawed design decisions of the language. The CSS Working Group at the W3C is responding by improving the power of CSS and the native web platform. It will be interesting to see the outcome…

The latest ways to deal with the cascade, inheritance and specificity originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

8 simple rules for a robust, scalable CSS architecture

Chris Coyier — Mon, 31 Oct 2016 23:35:15 +0000

I don’t disagree on any particular point on this thesis by Jarno Rantanen.

This is the first I’ve seen this particular naming convention, which seems fine to me, but I’d add that any well-considering naming convention works.

Also, there is this:

Cascading styles will ruin your day, eventually.

A sentiment shared by many these days, and the likely culprit for all the peter-griffin-adjusting-the-blinds.gifs out there in the world. Again I don’t entirely disagree, but, there are styles I gotta imagine even hardened CSS module aficionados would allow to cascade. For instance: why wouldn’t I let the body copy font-family cascade?

To Shared Link — Permalink on CSS-Tricks

8 simple rules for a robust, scalable CSS architecture originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

cascade – CSS-Tricks

Taming the Cascade With BEM and Modern CSS Selectors

What’s this about modern CSS selectors?

Dang. So now what?

:where() allows us to nest

What about non-BEM HTML?

That’s it!

Don’t Fight the Cascade, Control It!

O cascade, :where art thou?

Example: A configurable table component

CSS Custom Properties

Adding parameters with another data-attribute

Cascade Layers

Now you’re in control!

Cascade Layers

How are we going to use this?

Debugging

Browser Support

Desktop

Mobile / Tablet

Updates

Should DevTools teach the CSS cascade?

Computed Values: More Than Meets the Eye

Crash course on inheritance and the cascade

initial

inherit

unset

revert

Finding computed values in DevTools

Values and how they’re processed

Declared values

Cascaded values

Specified values

Computed values

Used values

Actual values

Inheritance in browser style computations

Color inheritance

Font size inheritance

Percentage-specified font size inheritance

Forced inheritance and post-layout computation

Properties that inherit by default

Further reading

Using Custom Property “Stacks” to Tame the Cascade

A quick intro to Custom Properties

Building variable “stacks”

Defining “scope”

Proximity scopes

Custom Properties and proximity

Component ownership

Defining “origins”

Mix and match

The Order of CSS Classes in HTML Doesn’t Matter

The “C” in CSS: The Cascade

The cascade cares about how and where styles are written

Elements can inherit styles from other elements

SVG inline attributes

Stylesheets

Specificity or selectors

Ordering

Inline styles

The important keyword

Overriding inline rules

Overriding specificity rules

Overriding ordering rules

Visualizing the cascade

CSS Basics: The Second “S” in CSS

The Sheet Contains the Styles

The Sheet is a Digital File

The Sheet is Linked Up to the HTML

The Sheet is Not Required for HTML

A Headline

The Sheet is Important

Leveling Up

The latest ways to deal with the cascade, inheritance and specificity

The :is() Pseudo-Class

Shadow DOM

Desktop

Mobile / Tablet

The all Property

`:where()` allows us to nest

O cascade, `:where` art thou?

`initial`

`inherit`

`unset`

`revert`

The `:is()` Pseudo-Class

The `all` Property