jetpack-boost-critical-css-gen

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.

Warning

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

Basic API

const { BrowserInterfacePlaywright, 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 BrowserInterfacePlaywright or 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 with an IFrame:

  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 );
      },
    },
  } );

Example usage in TypeScript with Playwright:

import { chromium, Page } from 'playwright';
import { BrowserInterfacePlaywright, generateCriticalCSS } from 'jetpack-boost-critical-css-gen';

async function playwrightGenerator( urls: string[] ): Promise< string > {
	const browser = await chromium.launch();
	const context = await browser.newContext();

	// Open playwright pages for each URL.
	const pages: { [ url: string ]: Page } = {};
	for ( const url of urls ) {
		pages[ url ] = await context.newPage();
		await pages[ url ].goto( url );
	}

	// Call the Critical CSS generator.
	const [ css, warnings ] = await generateCriticalCSS( {
		urls,
		viewports: [
			{ width: 640, height: 480 },
			{ width: 1024, height: 768 },
			{ width: 1280, height: 1024 },
		],
		browserInterface: new BrowserInterfacePlaywright( pages ),
	} );

	if ( warnings.length ) {
		console.warn( warnings );
	}

	return css;
}

Releasing new version

• Update the const version in src/index.ts
• Update version number in package.json
• Commit your changes
• Create a new release in jetpack-boost-critical-css-gen
  • Create a new tag in the release with the release version with the format release-x.x.x
  • The release title will be the same as the tag.
  • Click "Generate release notes" to automatically generate new release note.
  • Publish

Local development

Since we generally do development in docker, it is hard to test this with Jetpack Boost. npm link does not work in this case. To get around this issue, we can use yalc and do some customization to make development and testing easy. Here is what to do:

• Run npm -g yalc to install yalc globally.
• Run yalc publish from this package directory to publish this locally as a yalc repository.
• Go to your jetpack boost folder (jetpack/projects/packages/boost).
• Run yalc link jetpack-boost-critical-css-gen to link from this the dependency to this library.
• Now every time you want to sync changes, use yalc push from this library.

See: pc9hqz-1NI-p2