How to actually preload fonts

The best way to avoid layout shifts that occur because of font loading delays is to serve them from your own domain and preload them. The thing is, doing it correctly is kinda unintuitive. You’d think doing this would work:

<link
  rel="preload"
  href="/fonts/MerchantREGRound.woff2"
  as="font"
  type="font/woff2"
/>

<style>
  @font-face {
    font-family: "Merchant";
    font-weight: 400;
    src: url("/fonts/MerchantREGRound.woff2");
  }
</style>

You’ve got your Webfont files served from your domain, which lets the browser save on one TCP round-trip (not to mention you’ve stopped Google from snooping on your visitors), but this doesn’t quite work:

I'm reloading a browser window with 4 big bold words and two buttons. As I do, the buttons jump around in the first 300ms of the reload.

All I’m doing in that video is reloading the page. Watch the New and Join buttons on the bottom right, see how they jump around? That’s happening because the fallback font I use doesn’t have the same letter-spacing as my fancy, custom font.

The issue isn’t that the preload isn’t fast enough, it’s that the preload isn’t happening at all. You need a crossorigin attribute on your preload <link> tag to actually kick it off:

<link
  rel="preload"
  href="/fonts/MerchantREGRound.woff2"
  as="font"
  type="font/woff2"
  crossorigin
/>

And now, no intial layout shift:

The same browser window as before, but without the buttons jumping around

The crossorigin attribute tells the browser to send over CORS headers with this request. As far as I can tell, the issue doesn’t have anything to do with CORS since we’re preloading from the same origin, but instead with browsers requiring that font preloads to have CORS headers. With no value, the crossorigin attribute defaults to same-origin, and forces CORS headers.