Critters is a plugin that inlines your app's critical CSS and lazy-loads the rest.

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

Critters' 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.

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

First, install Critters as a development dependency:

npm i -D @playform/critters

or

yarn add -D @playform/critters

import Critters from "@playform/critters";

const critters = new Critters({
	// optional configuration (see below)
});

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

const inlined = await critters.process(html);

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

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

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

• options

• 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)
• 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](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/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)
• 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)
• logLevel String Controls log level of the plugin (default: "info")
• logger object Provide a custom logger interface logger
• includeSelectors RegExp | String Provide a list of selectors that should be included in the critical CSS. Accepts both RegExp and string.

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

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

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

/* critters: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

/* critters:exclude start */

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

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

/* critters:exclude end */

/* critters:include start */

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

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

/* critters:include end */

By default Critters evaluates the CSS against the entire input HTML. Critters evaluates the Critical CSS by reconstructing the entire DOM and evaluating the CSS selectors to find matching nodes. Usually this works well as Critters 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. Critters 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 Critters makes use of Critters containers.

A Critters container mimics the viewport and can be enabled by adding data-critters-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-critters-container > around the contents.

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

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

Custom logger interface:

Type: object

• 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

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")

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
• "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" | "js" | "js-lazy")

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
• react-snap

See CHANGELOG.md for a history of changes to this integration.