chrome-critical-css v1.0.3

ūsus

Webpage pre-rendering service. ⚡️

Features

• Renders webpage using the Chrome Debugging Protocol (CDP).
• Extracts CSS used to render the page.
• Renders HTML with the blocking CSS made asynchronous.
• Inlines the critical CSS.
• Preloads CSS and fonts used to render the page using rel=preload.

• Motivation
  • Demo
  • Use cases
• Node.js API
  • Configuration
• Installation
  • Dependencies
• Cookbook
  • Using via the command line interface (CLI)
  • Building Docker container with Chrome
  • Minify the CSS
  • Debugging
• Implementation
• Alternatives

Motivation

Static HTML pages with inline CSS load faster and are better indexed than single page applications (SPA). ūsus pre-renders single page applications into static HTML with the critical CSS inlined.

Removing the blocking CSS and inlining the CSS required to render the page increases the perceived page loading speed. Presumably, improves SEO by reducing the page loading time.

Read Pre-rendering SPA for SEO and improved perceived page loading speed.

Demo

Examples of web pages using ūsus:

• https://go2cinema.com/movies/baywatch-2017-1198354
• https://go2cinema.com/venues/odeon-leicester-square-1001206

Use cases

• Produce HTML used to render the page. Used to render single page applications (e.g. React and Angular) to produce a static HTML. This can be used as a replacement of https://prerender.io/. Default behaviour.
• Extract CSS used to render a specific page. Used to capture the critical CSS. Use --extractStyles option.
• Produce HTML used to render the page with the critical-path CSS inlined and blocking CSS made asynchronous. Use --inlineStyles option.

Node.js API

ūsus can be used either as a Node.js dependency or as a CLI program.

import {
  render
} from 'usus';

/**
 * @see https://github.com/gajus/usus#configuration
 */
const configuration: UserConfigurationType = {}

const css = await render('http://gajus.com/', configuration);

Configuration

// Flow type annotations included for user reference only.
// ūsus does not depend or require use of Flow type.
//
// Refer to the table below for an alternative form of documentation.

type CookieType = {|
  +name: string,
  +value: string
|};

export type UserDeviceMetricsOverrideType = {
  +deviceScaleFactor?: number,
  +fitWindow?: boolean,
  +height?: number,
  +mobile?: boolean,
  +width?: number
};

type FormatStylesType = (styles: string) => Promise<string>;

export type UserConfigurationType = {
  +chromePort?: number,
  +cookies?: $ReadOnlyArray<CookieType>,
  +delay?: number,
  +deviceMetricsOverride?: UserDeviceMetricsOverrideType,
  +extractStyles?: boolean,
  +formatStyles?: FormatStylesType,
  +inlineStyles?: boolean,
  +preloadFonts?: boolean,
  +preloadStyles?: boolean
};

The default behaviour is to return the HTML.

• Using the --extractStyles option returns the CSS used to render the document.
• Using the --inlineStyles option returns HTML document with CSS inlined.

deviceMetricsOverride configuration

For more information about the deviceMetricsOverride configuration, refer to Chrome DevTools Protocol Viewer documentation.

Installation

Using ūsus requires to install usus NPM package and Google Chrome browser (refer to Dependencies).

Dependencies

ūsus depends on Chrome v59+.

For Docker installation instructions, refer to Building Docker container with Chrome.

Cookbook

Using via the command line interface (CLI)

$ npm install usus --global
$ usus --help
# usus <command> --help
$ usus render --help
# Renders static HTML. Equivalent to https://prerender.io/.
$ usus render --url http://gajus.com/
# Inlines styles required to render the page.
$ usus render --url http://gajus.com/ --inlineStyles true
# Use cookies when loading the page.
$ usus render --url http://gajus.com/ --cookies foo=bar,baz=qux
# Render emulating a mobile device (example is using iPhone 6 parameters).
$ usus render --url http://gajus.com/ --deviceMetricsOverride.deviceScaleFactor 2 --deviceMetricsOverride.fitWindow false --deviceMetricsOverride.height 1334 --deviceMetricsOverride.mobile true --deviceMetricsOverride.width 750

Building Docker container with Chrome

Add the following line to your Dockerfile:

RUN wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | apt-key add - \
  && sh -c 'echo "deb http://dl.google.com/linux/chrome/deb/ stable main" >> /etc/apt/sources.list.d/google.list' \
  && apt-get update -y \
  && apt-get install google-chrome-stable -y

This assumes that you are extending from the base node image.

Controlling the Chrome instance

By default, ūsus creates a new instance of Chrome for every render operation and destroys it after completion. However, you can start Chrome independent of ūsus and re-use the same instance for multiple renderings.

import {
  launchChrome,
  render
} from 'usus';

const chrome = await launchChrome();

await render('https://go2cinema.com/movies/baywatch-2017-1198354', {
  chromePort: chrome.port,
  inlineStyles: true
});

await render('https://go2cinema.com/movies/baby-driver-2017-2257838', {
  chromePort: chrome.port,
  inlineStyles: true
});

await chrome.kill();

launchChrome is a convenience method to launch Chrome using default ūsus configuration. If you need granular control over how Chrome is launched, refer to the chrome-launcher program.

Minifying the CSS

Use the formatStyles callback to minify/ format/ optimize/ remove CSS before it is inlined.

In this example, I am using csso minifier.

import {
  render
} from 'usus';
import {
  minify
} from 'csso';

await render(url, {
  formatStyles: (styles: string): Promise<string> => {
    return minify(styles).css;
  },
  inlineStyles: true
});

Debugging

Export DEBUG=usus variable to get additional debugging information, e.g.

$ export DEBUG=usus*
$ usus --url http://gajus.com

Implementation

ūsus uses Chrome Debugging Protocol CSS coverage report to generate a stylesheet used to render the document.

Alternatives

The following programs provide equivalent service:

• https://github.com/giakki/uncss
• https://github.com/pocketjoso/penthouse
• https://github.com/addyosmani/critical
• https://github.com/filamentgroup/criticalcss

All of these programs are using PhantomJS or JSDom to render the page/ evaluate the scripts.

ūsus is different because it is using Chrome Debugging Protocol and it leverages the Chrome CSS coverage report to generate a stylesheet used to render the document.