Feb 12, 2021

Google Web Vitals and Cumulative Layout Shift (CLS)

By Mastering JS Weekly • Issue #57 • View online

Google Web Vitals was a major change to how Google ranks search results based on measurable website quality. It was released in late 2020 and Mastering JS’ traffic immediately took a hit. Here’s why, and what we’ve done about it.

Cumulative Layout Shift

CLS measures how much your page layout changes as your page is loading. Imagine a <div> tag that shifts down when a big image is loaded. That’s an example of a layout shift.

The "Lorem ipsum" div shifts down, which increases CLS and decreases SEO

One common reason for CLS is inline ads. When the ad loads, it shifts the rest of the layout down. The solution is simple: wrap the banner in a fixed height div so the banner doesn’t shift the rest of the page down.

Google's recommendation in one picture

Font Fallbacks Matter

If you’re loading fonts via <link> tags, that can affect your CLS. For example, Mastering JS loads Montserrat for copy and Inconsolata for code samples from Google Fonts. Montserrat is a fairly large font: 16px Montserrat is bigger than 16px sans-serif.

Layout with Sans-Serif

Layout with Montserrat

The layout shift caused by Montserrat loading was significantly increasing our measured CLS. Although most users aren’t affected by this layout shift because Google Fonts is pretty fast, we still need to fix this for SEO.

There are ways to preload fonts in more recent versions of Google, but we came up with a neat alternative: change the fallback font. DejaVu Sans is much closer to Montserrat, which means minimal layout shift and a near-zero CLS with no special browser features or extra JavaScript.

Layout with DejaVu Sans

The key takeaway here is that fallback fonts aren’t just for show anymore. Picking the right fallback font can drastically reduce your CLS and improve your SEO.

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