The Almost-Complete Guide to Cumulative Layout Shift

Here’s Jess B. Peck writing all about Google’s Core Web Vitals:

Let’s step back one. CLS is when you’re about to click on a link, and the whole page shifts and you click on a different link instead. It’s when you’re halfway through a blogpost and an ad loads and you lose your place. It is when… the layout shifts. At least, that’s what it’s trying to measure– both those shifts, how often they happen, and the irritation that causes the user.

I didn’t quite understand just how complex Cumulative Layout Shift is before reading Jess’s piece. As Jess explains:

CLS is a measure for a robot to approximate the user perception of instability. This means we’re getting a unit of change over time. It’s a three dimensional equation, and there are tons of things that can affect it. […] The idea is more to alert devs to a problem area, rather than be a perfect measurement of how annoying a page is.

I had this problem on, of all places, Google dot com. I kept tapping an element just as it appeared on screen and this sent me to the wrong page.

Jess notes that these metrics are sometimes more of an art than a science, and so we shouldn’t be obsessed with making sure that just these Core Web Vital metrics are okay. Chris mentioned a while ago that he worries folks might begin to game these metrics for improving their SEO:

This feels like the start of a weird new era of web performance where the metrics of web performance have shifted to user-centric measurements, but people are implementing tricky strategies to game those numbers with methods that, if anything, slightly harm user experience.

Harry Roberts mentioned something similar:

A new and unusual phenomenon: clients reluctant (even refusing) to fix performance issues unless they directly improve Vitals.

— Harry Roberts (@csswizardry) March 5, 2021

I feel like this is our responsibility as web developers, to explain that what we want to do here is reduce user misery on our websites. That’s not to say it’s easy, though, and there’s certainly not much we can do to avoid the shady folks who’ll game these metrics only to improve SEO.

As Jeremy wrote just the other day:

The map is not the territory. The numbers are a proxy for user experience, but it’s notoriously difficult to measure intangible ideas like pain and frustration.

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