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Optimizing web fonts loading for CLS (Cumulative Layout Shift)

When we use external web fonts in our web pages we add a new network request that needs to be resolve to fully render the text. This can lead to a situation where the text changes position after the font is loaded. See the below picture.

This will happen if we import our fonts directly in the CSS files:

/* the wrong way */
@import url('https://fonts.googleapis.com/css2?family=Montserrat:wght@400&family=Roboto:wght@300&display=swap');

We can improve this experience by importing the fonts in the head tag of the document.

<!-- do this instead -->
<link rel="preconnect" href="https://fonts.gstatic.com">
<link href="https://fonts.googleapis.com/css2?family=Roboto:wght@100&display=swap" rel="stylesheet">

Note the rel="preconnect". Adding it will tell the browser to prioritize an early connection to that resource. However, this will mean that other requests will be delayed. So use preconnect with care.

We can go one sept forward by adding the text query param in the request URL of the web font. This will ensure the only the requested characters will be loaded, instead of the full set of characters in the web font. In our case, it will load only the H e l and o characters. More details here.

<!-- this is even better -->
<link rel="preconnect" href="https://fonts.gstatic.com">
<link href="https://fonts.googleapis.com/css2?family=Roboto:wght@100&display=swap&text=Hello" rel="stylesheet">

I have found out about this cool performance trick from this article written by Katie Hempenius and Una Kravets. Kudos to them!

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