sui-critical-css

Extract Critical CSS from a set of URLs for an app

How it works

1. Read the config options and routes provided.
2. For each route, it opens a browser, navigate and extract the Critical CSS.
3. Create a css file in the critical-css folder.
4. After doing this for each route, then creates a critical.json file that could be read for every path to extract the critical-css.
5. Use then @s-ui/critical-css-middleware to extract to use in your Express app the CSS.

How to use to extract

Install package to your project:

npm install @s-ui/critical-css -D

Programmatic usage:

In order to extract critical css and match extracted files with your page or route you can use two approaches:

• Using path-to-regex
• Using page component displayName

You can combine both of them.

Additionally there are two optional config parameters:

• requiredClassNames: A list of required css class names. If they aren't present in the generated Critical CSS, it would be discarded. By default there would be 2 retries to try to get the correct Critical CSS
• retries: Number of retries if the requiredClassNames aren't present in the Critical CSS. By default it's 2.

Using path-to-regex:

You can use Express Route Tester to create and validate that your Path-to-Regexp works as expected.

Example:

// scripts/get-critical-css-for-routes.js
import {extractCSSFromApp} from '@s-ui/critical-css'

const config = {
  hostname: 'http://localhost'
}

const routes = {
  '/:lang': {
    url: '/es'
  },
  '/:lang/catalogo-productos': {
    url: ['/es/catalogo-productos', '/en/catalogo-productos']
  }
}

extractCSSFromApp({config, routes})

Using displayName

Example:

// scripts/get-critical-css-for-routes.js
import {extractCSSFromApp} from '@s-ui/critical-css'

// Page display names
const displayNames = {
  home: 'Home',
  list: 'List'
}

const config = {
  hostname: 'http://localhost'
}

const routes = {
  [displayNames.home]: {
    url: '/es'
  },
  [displayNames.list]: {
    url: '/es/catalogo-productos',
    requiredClassNames: ['.ma-AdCardV2'],
    retries: 3
  }
}

extractCSSFromApp({config, routes})