Critical CSS Generator built for Jetpack Boost. It can generate Critical CSS on the server-side using Puppeteer, or on the client-side using iframes. It also supports generating blocks of Critical CSS that can apply to multiple URLs, and/or multiple viewports.

This is a work in progress, and its API is not guaranteed to be stable. :)

const { BrowserInterfacePuppeteer, BrowserInterfaceIframe, generateCriticalCSS } = require( 'jetpack-boost-critical-css-gen' );
const [css, warnings] = generateCriticalCSS( options );

Where options is an object with the following keys:

• urls (required) - An array of URLs to generate Critical CSS for.
• viewports (required) - An array of viewport sizes to generate Critical CSS for. Each entry should be an object with a width and height property.
• browserInterface (required) - An instance of either BrowserInterfacePuppeteer or BrowserInterfaceIframe which defines an interface for the Generator to query the (real or virtual) browser.
• progressCallback - A callback to receive progress information, with two arguments; step and stepCount. Each are integers; progress percentage can be expressed in the form percentage = step * 100 / stepCount.
• filters - An object describing a filter to run each property and/or atRule through for inclusion.

Example usage:

  const { BrowserInterfaceIframe, generateCriticalCSS } = require( 'jetpack-boost-critical-css-gen' );
  
  const [css, warnings] = await generateCriticalCSS( {
    urls: [ 'http://example.com' ],
    viewports: [ { width: 800, height: 600 } ],
    progressCallback: ( step, stepCount ) => { console.log( `Step ${ step } of ${ stepCount }.` ); },
    browserInterface: new BrowserInterfaceIframe( {} ),
    filters: {
      properties: ( key, value ) => {
        return ! /^\s*url\s*\(/.test( value );
      },
    },
  } );