Logo of fontaine


Automatic font fallback based on font metrics

Automatic font fallback based on font metrics


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

On the playground project, enabling/disabling fontaine makes the following difference rendering /, with no customisation required:



With pnpm

pnpm add -D fontaine

Or, with npm

npm install -D fontaine

Or, with yarn

yarn add -D fontaine


import { FontaineTransform } from 'fontaine'

// Astro config - astro.config.mjs
import { defineConfig } from 'astro/config'

const options = {
  fallbacks: ['BlinkMacSystemFont', 'Segoe UI', 'Helvetica Neue', 'Arial', 'Noto Sans'],
  // You may need to resolve assets like `/fonts/Roboto.woff2` to a particular directory
  resolvePath: id => `file:///path/to/public/dir${id}`,
  // overrideName: (originalName) => `${name} override`
  // sourcemap: false
  // skipFontFaceGeneration: (fallbackName) => fallbackName === 'Roboto override'

// Vite
export default {
  plugins: [FontaineTransform.vite(options)]

// Next.js
export default {
  webpack(config) {
    config.plugins = config.plugins || []
    return config

// Docusaurus plugin - to be provided to the plugins option of docusaurus.config.js
// n.b. you'll likely need to require fontaine rather than importing it
const fontaine = require('fontaine')

function fontainePlugin(_context, _options) {
  return {
    name: 'fontaine-plugin',
    configureWebpack(_config, _isServer) {
      return {
        plugins: [

// Gatsby config - gatsby-node.js
const { FontaineTransform } = require('fontaine')

exports.onCreateWebpackConfig = ({ stage, actions, getConfig }) => {
  const config = getConfig()

export default defineConfig({
  integrations: [],
  vite: {
    plugins: [
        fallbacks: ['Arial'],
        resolvePath: id => new URL(`./public${id}`, import.meta.url), // id is the font src value in the CSS

Note If you are using Nuxt, check out nuxt-font-metrics which uses fontaine under the hood.

If your custom font is used through the mechanism of CSS variables, you'll need to make a tweak to your CSS variables to give fontaine a helping hand. Docusaurus is an example of this, it uses the --ifm-font-family-base variable to reference a custom font. In order that fontaine can connect the variable with the font, we need to add a {Name of Font} override suffix to that variable. What does this look like? Well imagine we were using the custom font Poppins which is referenced from the --ifm-font-family-base variable, we'd make the following adjustment:

:root {
  /* ... */
-  --ifm-font-family-base: 'Poppins';
+  --ifm-font-family-base: 'Poppins', 'Poppins override';

Behind the scenes, there is a 'Poppins override' @font-face rule that has been created by fontaine. By manually adding this override font family to our CSS variable, we make our site use the fallback @font-face rule with the correct font metrics that fontaine generates.

How it works

fontaine 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')
  font-weight: 700;
/* This additional font-face declaration will be added to your CSS. */
@font-face {
  font-family: 'Roboto override';
  src: local('BlinkMacSystemFont'), local('Segoe UI'), 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', fontaine will add the override to the font-family:

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

💻 Development

  • Clone this repository
  • Enable Corepack using corepack enable (use npm i -g corepack for Node.js < 16.10)
  • Install dependencies using pnpm install
  • Run interactive tests using pnpm dev; launch a vite server using source code with pnpm demo:dev


This would not have been possible without:


Made with ❤️

Published under MIT License.