beasties

Beasties

Beasties is a plugin that inlines your app's critical CSS and lazy-loads the rest. It is a maintained fork of GoogleChromeLabs/critters

beasties

It's a little different from other options, because it doesn't use a headless browser to render content. This tradeoff allows Beasties to be very fast and lightweight. It also means Beasties inlines all CSS rules used by your document, rather than only those needed for above-the-fold content. For alternatives, see Similar Libraries.

Beasties' design makes it a good fit when inlining critical CSS for prerendered/SSR'd Single Page Applications. It was developed to be an excellent compliment to prerender-loader, combining to dramatically improve first paint time for most Single Page Applications.

Features

• Fast - no browser, few dependencies
• Integrates with Webpack beasties-webpack-plugin
• Supports preloading and/or inlining critical fonts
• Prunes unused CSS keyframes and media queries
• Removes inlined CSS rules from lazy-loaded stylesheets

Installation

First, install Beasties as a development dependency:

npm i -D beasties

or

yarn add -D beasties

Simple Example

import Beasties from 'beasties'

const beasties = new Beasties({
  // optional configuration (see below)
})

const html = `
  <style>
    .red { color: red }
    .blue { color: blue }
  </style>
  <div class="blue">I'm Blue</div>
`

const inlined = await beasties.process(html)

console.log(inlined)
// "<style>.blue{color:blue}</style><div class=\"blue\">I'm Blue</div>"

Usage with Vite

Beasties can be used with Vite through vite-plugin-beasties.

Just add it to your Vite configuration:

// vite.config.ts
import { defineConfig } from 'vite'
import { beasties } from 'vite-plugin-beasties'

export default defineConfig({
  plugins: [
    beasties({
      // optional beasties configuration
      options: {
        preload: 'swap',
      }
    })
  ]
})

The plugin will process the output for your index.html and inline critical CSS while lazy-loading the rest.

Usage with webpack

Beasties is also available as a Webpack plugin called beasties-webpack-plugin.

The Webpack plugin supports the same configuration options as the main beasties package:

// webpack.config.js
+const Beasties = require('beasties-webpack-plugin');

module.exports = {
  plugins: [
+    new Beasties({
+      // optional configuration
+      preload: 'swap',
+    })
  ]
}

That's it! The resultant html will have its critical CSS inlined and the stylesheets lazy-loaded.

Usage

Beasties

All optional. Pass them to new Beasties({ ... }).

Parameters

• options

Properties

• path String Base path location of the CSS files (default: '')
• publicPath String Public path of the CSS resources. This prefix is removed from the href (default: '')
• external Boolean Inline styles from external stylesheets (default: true)
• remote Boolean Download and inline remote stylesheets (http://, https://, //) (default: false)
• inlineThreshold Number Inline external stylesheets smaller than a given size (default: 0)
• minimumExternalSize Number If the non-critical external stylesheet would be below this size, just inline it (default: 0)
• pruneSource Boolean Remove inlined rules from the external stylesheet (default: false)
• mergeStylesheets Boolean Merged inlined stylesheets into a single <style> tag (default: true)
• additionalStylesheets Array<String> Glob for matching other stylesheets to be used while looking for critical CSS.
• reduceInlineStyles Boolean Option indicates if inline styles should be evaluated for critical CSS. By default inline style tags will be evaluated and rewritten to only contain critical CSS. Set it to false to skip processing inline styles. (default: true)
• allowRules Array<String | RegExp> Always include rules matching these selectors or patterns in the critical CSS, regardless of whether they match elements in the document. (default: [])
• preload String Which preload strategy to use
• noscriptFallback Boolean Add <noscript> fallback to JS-based strategies
• inlineFonts Boolean Inline critical font-face rules (default: false)
• preloadFonts Boolean Preloads critical fonts (default: true)
• fonts Boolean Shorthand for setting inlineFonts + preloadFonts* Values:
  • true to inline critical font-face rules and preload the fonts
  • false to don't inline any font-face rules and don't preload fonts
• keyframes String Controls which keyframes rules are inlined.* Values:
  • "critical": (default) inline keyframes rules used by the critical CSS
  • "all" inline all keyframes rules
  • "none" remove all keyframes rules
• compress Boolean Compress resulting critical CSS (default: true)
• safeParser Boolean Use PostCSS safe parser for fault-tolerant CSS parsing. Handles legacy code with syntax errors (default: true)
• logLevel String Controls log level of the plugin (default: "info")
• logger object Provide a custom logger interface logger

Include/exclude rules

We can include or exclude rules to be part of critical CSS by adding comments in the CSS

Single line comments to include/exclude the next CSS rule

/* beasties:exclude */
.selector1 {
  /* this rule will be excluded from critical CSS */
}

.selector2 {
  /* this will be evaluated normally */
}

/* beasties:include */
.selector3 {
  /* this rule will be included in the critical CSS */
}

.selector4 {
  /* this will be evaluated normally */
}

Including/Excluding multiple rules by adding start and end markers

/* beasties:exclude start */

.selector1 {
  /* this rule will be excluded from critical CSS */
}

.selector2 {
  /* this rule will be excluded from critical CSS */
}

/* beasties:exclude end */

/* beasties:include start */

.selector3 {
  /* this rule will be included in the critical CSS */
}

.selector4 {
  /* this rule will be included in the critical CSS */
}

/* beasties:include end */

Programmatically including rules with allowRules

In addition to comment-based inclusion, you can use the allowRules option to programmatically include specific selectors or patterns in the critical CSS, regardless of whether they match elements in the document. This is useful for cases where you know certain styles should always be included.

const beasties = new Beasties({
  // Always include these selectors in critical CSS
  allowRules: [
    // Exact selector match
    '.always-include',
    // Regular expression pattern
    /^\.modal-/
  ]
})

With this configuration, any CSS rules with selectors that match .always-include exactly or start with .modal- will be included in the critical CSS, even if no matching elements exist in the document.

Beasties container

By default Beasties evaluates the CSS against the entire input HTML. Beasties evaluates the Critical CSS by reconstructing the entire DOM and evaluating the CSS selectors to find matching nodes. Usually this works well as Beasties is lightweight and fast.

For some cases, the input HTML can be very large or deeply nested which makes the reconstructed DOM much larger, which in turn can slow down the critical CSS generation. Beasties is not aware of viewport size and what specific nodes are above the fold since there is not a headless browser involved.

To overcome this issue Beasties makes use of Beasties containers.

A Beasties container mimics the viewport and can be enabled by adding data-beasties-container into the top level container thats contains the HTML elements above the fold.

You can estimate the contents of your viewport roughly and add a <div data-beasties-container > around the contents.

<html>
  <body>
    <div class="container">
      <div data-beasties-container>
        /* HTML inside this container are used to evaluate critical CSS */
      </div>
      /* HTML is ignored when evaluating critical CSS */
    </div>
    <footer></footer>
  </body>
</html>

You can mark more than one element with data-beasties-container. CSS is then inlined if it matches inside any of the containers, which is useful when the above-the-fold content is spread across multiple disconnected regions.

<html>
  <body>
    <header data-beasties-container>
      /* evaluated */
    </header>
    <main>
      /* ignored */
    </main>
    <aside data-beasties-container>
      /* evaluated */
    </aside>
  </body>
</html>

Note: This is an easy way to improve the performance of Beasties

Skipping individual stylesheets

If a stylesheet should not be processed by Beasties — for example a file that is injected or replaced at runtime (e.g. multi-tenant theming via Docker volume mounts) — add the data-beasties-skip boolean attribute to its <link> tag:

<!-- processed normally -->
<link rel="stylesheet" href="styles.css">

<!-- skipped entirely — tag is left untouched in the final HTML -->
<link rel="stylesheet" href="custom-theme.css" data-beasties-skip>

Beasties will not read, inline, prune, or mutate that tag regardless of the active preload strategy. This is useful when disabling inlineCritical globally would sacrifice Core Web Vitals optimizations for the rest of your stylesheets.

Logger

Custom logger interface:

Type: object

Properties

• trace function (String) Prints a trace message
• debug function (String) Prints a debug message
• info function (String) Prints an information message
• warn function (String) Prints a warning message
• error function (String) Prints an error message

LogLevel

Controls log level of the plugin. Specifies the level the logger should use. A logger will not produce output for any log level beneath the specified level. Available levels and order are:

• "info" (default)
• "warn"
• "error"
• "trace"
• "debug"
• "silent"

Type: ("info" | "warn" | "error" | "trace" | "debug" | "silent")

PreloadStrategy

The mechanism to use for lazy-loading stylesheets.

Note: JS indicates a strategy requiring JavaScript (falls back to <noscript> unless disabled).

• default: Move stylesheet links to the end of the document and insert preload meta tags in their place.
• "body": Move all external stylesheet links to the end of the document.
• "media": Load stylesheets asynchronously by adding media="not x" and removing once loaded. JS
• "swap": Convert stylesheet links to preloads that swap to rel="stylesheet" once loaded (details). JS
• "swap-high": Use <link rel="alternate stylesheet preload"> and swap to rel="stylesheet" once loaded (details). JS
• "swap-low": Use <link rel="alternate stylesheet"> (no preload in rel here!) and swap to rel="stylesheet" once loaded (details). It ensures lowest priority compared to swap and swap-high. JS
• "js": Inject an asynchronous CSS loader similar to LoadCSS and use it to load stylesheets. JS
• "js-lazy": Like "js", but the stylesheet is disabled until fully loaded.
• false: Disables adding preload tags.

Type: (default | "body" | "media" | "swap" | "swap-high" | "swap-low" | "js" | "js-lazy")

Similar Libraries

There are a number of other libraries that can inline Critical CSS, each with a slightly different approach. Here are a few great options:

• Critical
• Penthouse
• webpack-critical
• webpack-plugin-critical
• html-critical-webpack-plugin
• react-snap

License

Apache 2.0

This is not an official Google product.