Font metric fallback implementation for Nuxt 3

• ✨  Changelog
• ▶️  Online playground

⚠️ @nuxtjs/fontaine is under active development. ⚠️

• 💪 Reduces CLS by using local font fallbacks with crafted font metrics.
• ✨ Generates font metrics and fallbacks automatically.
• ⚡️ Pure CSS, zero runtime overhead.

On the playground project, enabling/disabling this module makes the following differences rendering /, with no customisation required:

For best performance, you will need to inline all your CSS, not just the font-face fallback rules (which this module does automatically), or there will still be a layout shift when the stylesheet loads (which is why the number above is not zero).

This PR aims to bring that ability to Nuxt itself.

npx nuxi@latest module add fontaine

export default defineNuxtConfig({
  modules: ['@nuxtjs/fontaine'],
  // If you are using a Google font or you don't have a @font-face declaration
  // for a font you're using, you can declare them here.
  //
  // In most cases this is not necessary.
  //
  // fontMetrics: {
  //   fonts: ['Inter', { family: 'Some Custom Font', src: '/path/to/custom/font.woff2' }],
  // },
})

That's it!

If you use Tailwind CSS and use your custom font via the config file, you will need to manually add the fallback font.

import type { Config } from 'tailwindcss'
import { fontFamily } from 'tailwindcss/defaultTheme'

export default <Partial<Config>> {
  theme: {
    extend: {
      fontFamily: {
        sans: ['Roboto', 'Roboto fallback', ...fontFamily.sans],
      },
    },
  },
}

Nuxt will scan your @font-face rules and generate fallback rules with the correct metrics. For example:

@font-face {
  font-family: 'Roboto';
  font-display: swap;
  src: url('/fonts/Roboto.woff2') format('woff2'), url('/fonts/Roboto.woff') format('woff');
  font-weight: 700;
}
/* This will be generated. */
@font-face {
  font-family: 'Roboto fallback';
  src: local('BlinkMacSystemFont'), local('Segoe UI'), local('Roboto'), local('Helvetica Neue'),
    local('Arial'), local('Noto Sans');
  ascent-override: 92.7734375%;
  descent-override: 24.4140625%;
  line-gap-override: 0%;
}

Then, whenever you use font-family: 'Roboto', Nuxt will add the fallback to the font-family:

:root {
  font-family: 'Roboto';
  /* This becomes */
  font-family: 'Roboto', 'Roboto fallback';
}

The core of this module will work outside of Nuxt, and has been separated into a separate library: fontaine. Check it out!

• Clone this repository
• Enable Corepack using corepack enable
• Install dependencies using pnpm install
• Stub module with pnpm dev:prepare
• Run pnpm dev to start playground in development mode

This would not have been possible without:

• amazing tooling and generated metrics from capsizecss
• suggestion and algorithm from Katie Hempenius & Kara Erickson on the Google Aurora team - see notes on calculating font metric overrides.

Made with ❤️

Published under the MIT License.