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2.1.0 • Public • Published


Get all the styles, you have used to render a page.

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Bundler and framework independent CSS part of SSR-friendly code splitting

Detects used css files from the given HTML, and/or inlines critical styles. Supports sync or stream rendering.

Code splitting

This is all about code splitting, Server Side Rendering and React, even if React has nothing to do with this library.

Code splitting is a good feature, and SSR is also awesome, but then you have to load all the used scripts on the client, before making a page alive. Everybody is talking not about .css, but only about .js.

That's done, in a different ways. That's not a big task, as long as the usage of code splitted blocks is trackable - you are using it, and components defined inside.

CSS is harder - you might just use random classes and what next? You are just importing CSS here and where, sometimes indirectly, and there is no way to understand what's happening.

While it's possible for webpack to add a Link to document header once some Component uses some Style, you can't do the same in the concurrent server environment - there is no to add a Link.

Code splitting libraries solved it straight forward - by building resource graph, and fetching all bound resources to that graph, but tracking is hard, and quite bound to the bundler, and could delay content sending.


  1. Scan all .css files, extracting all the style names.
  2. Scan resulting html, finding all the classNames used. 3a. Calculate all styles you need to render a given HTML. 3b. Calculate all the files you shall send to a client.
  3. Inject styles or links
  4. Hoist or remove styles on clientside startup

Bonus: Do the same for streams.

Bonus: Do it only for really used styled, not just imported somewhere.


In the performance sake used-styles inlines a bit more styles than it should - it's just harder to understand what shall be done.

  • inlines all @keyframe animations
  • inlines all html, body and other simple selectors (aka css-reset)
  • inlines all rules matching last part of a selector

And, hopefully

  • inlines all classes used in HTML code


Speed, I am speed!

For the 516kb page, which needs 80ms to renderToString resulting time for getCriticalRules(very expensive operation) would be around 4ms.


Discovery API

Use to scan your dist folder to create a look up table between classNames and files they are described in.

  1. discoverProjectStyles(buildDirrectory, [filter]): StyleDef - generates class lookup table

you may use the second argument to control which files should be scanned

filter is very important function here. It takes fileName as input, and returns false, true, or a number as result. False value would exclude chunk from the set, while number would change the order of the chunk. Keeping chunk ordering "as expected" is required to preserve style declaration order, which is important for many existing styles.

// with chunk format [chunkhash]_[id]
const styleData = discoverProjectStyles(resolve('build'), name => {
  // get ID of a chunk and use it as order hint
  const match = name.match(/(\d)_c.css/);
  return match && +match[1];


Use to get used styled from render result or a stream

  1. getUsedStyles(html, StyleDef): string[] - returns all used files, you have to import them

  2. getCriticalStyles(html, StyleDef) : string - returns all used selectors and other applicable rules, wrapped with style

  3. getCriticalRules(html, StyleDef): string - the same, but without <style> tag, letting you handle in a way you want

  4. createStyleStream(lookupTable, callback(fileName):void): TransformStream - creates Transform stream - will inject <links

  5. createCriticalStyleStream(lookupTable, callback(fileName):void): TransformStream - creates Transform stream - will inject <styles.


There are only two things about react:

  1. to inline critical styles use another helper - getCriticalRules which does not wrap result with style letting you do it
import {getCriticalRules} from 'used-styles';
const Header = () => (
       dangerouslySetInnerHTML={{__html:getCriticalRules(markup, styleData)}}
  1. React produces more valid code, and you might enable optimistic optimization, making used-styles a bit faster.
import {enableReactOptimization} from 'used-styles';
enableReactOptimization(); // 


Static rendering

There is nothing interesting here - just render, just getUsedStyles.

import {discoverProjectStyles, getUsedStyles} from 'used-styles';
// generate lookup table on server start
const stylesLookup = discoverProjectStyles('./build');
async function MyRender () {
  await stylesLookup;// it is "thenable"
  // render App
  const markup = ReactDOM.renderToString(<App />)
  const usedStyles = getUsedStyles(markup, stylesLookup);
usedStyles.forEach(style => {
  const link = `<link  rel="stylesheet" href="build/${style}">\n`;
  // or
  const link = `<link rel="prefetch" as="style" href="build/${style}">\n`;
  // append this link to the header output or to the body
// or 
const criticalCSS = getCriticalStyles(markup, stylesLookup);
// append this link to the header output

Any bulk CSS operations, both getCriticalStyles and getUsedStyles are safe and preserve the selector rule order. You may combine both methods, to prefetch full styles, and inline critical CSS.

! Keep in mind - calling two functions is as fast, as calling a single one !

Stream rendering

Please keep in mind - stream rendering in NOT SAFE in terms of CSS, as long as it might affect the ordering of selectors. Only pure BEM and Atomic CSS are "safe", just some CSS would not be compatible. Please test results before releasing into production.

If you do not understand why and how selector order is important - please do not use stream transformer.

Stream rendering is much harder, and much more efficient. The idea is to make it efficient, and not delay Time-To-First-Byte. And the second byte.

Stream rendering could be interleaved(more efficient) or block(more predictable).

Interleaved Stream rendering

In case or React rendering you may use interleaved streaming, which would not delay TimeToFirstByte. It's quite similar how StyledComponents works

import {discoverProjectStyles, createLink} from 'used-styles';
import {createStyleStream} from 'used-styles/react';
import MultiStream from 'multistream';
// generate lookup table on server start
const stylesLookup = discoverProjectStyles('./build'); // __dirname usually
// small utility for "readable" streams
const readable = () => {
  const s = new Readable();
  s._read = () => true;
  return s;
async function MyRender() {
  // render App
  const htmlStream = ReactDOM.renderToNodeStream(<App />)
  const lookup = await stylesLookup;
  // create a style steam
  const styledStream = createStyleStream(lookup, (style) => {
  // _return_ link tag, and it will be appended to the stream output
      return createLink(`dist/${style}`)
// or create critical CSS stream - it will inline all styles
const styledStream = createCriticalStyleStream(projectStyles);
  // allow client to start loading js bundle
  res.write(`<!DOCTYPE html><html><head><script defer src="client.js"></script>`);
  const middleStream = readableString('</head><body><div id="root">');
  const endStream = readableString('</head><body>');
  // concatenate all steams together
  const streams = [
      middleStream, // end of a header, and start of a body
    styledStream, // the main content
    endStream,    // closing tags
// start by piping react and styled transform stream

!! THIS IS NOT THE END !! Interleaving links and react output would produce break client side rehydration, as long as injected links are not rendered by React, and not expected to present in the "result" HTML code.

You have to move injected styles prior rehydration.

  import { moveStyles } from 'used-styles/moveStyles';

You might want to remove styles after rehydration to prevent duplication. Double check that corresponding real CSS is loaded.

  import { removeStyles } from 'used-styles/moveStyles';

Block rendering

Not sure this is a good idea

Idea is to:

  • push initial line to the browser, with the-main-script inside
  • push all used styles
  • push some html between styles and content
  • push content
  • push closing tags

That's all are streams, concatenated in a right order. It's possible to interleave them, but that's is not expected buy a hydrate.

import {discoverProjectStyles, createStyleStream, createLink} from 'used-styles';
import MultiStream from 'multistream';
// .....
// generate lookup table on server start
const lookup = await discoverProjectStyles('./build'); // __dirname usually
// small utility for "readable" streams
const readable = () => {
  const s = new Readable();
  s._read = () => true;
  return s;
// render App
const htmlStream = ReactDOM.renderToNodeStream(<App />)
// create a style steam
const styledStream = createStyleStream(lookup, (style) => {
    // emit a line to header Stream
    // or
    headerStream.push(`<link href="dist/${style}" rel="stylesheet">\n`);
// allow client to start loading js bundle
res.write(`<!DOCTYPE html><html><head><script defer src="client.js"></script>`);
const middleStream = readableString('</head><body><div id="root">');
const endStream = readableString('</head><body>');
// concatenate all steams together
const streams = [
    headerStream, // styles
    middleStream, // end of a header, and start of a body
    styledStream, // the main content
    endStream,    // closing tags
// start by piping react and styled transform stream
htmlStream.pipe(styledStream, {end: false});
htmlStream.on('end', () => {
    // kill header stream on the main stream end

This example is taken from Parcel-SSR-example from react-imported-component.

Hybrid usage

The advanced pattern described in Optimizing CSS Delivery article proposes to:

  • inline critical CSS for a first time customers
  • use cached .css files for recurring

This library does not provide a way to distinguish "one" cohort of customers from another, although, provides an API to optimize the delivery.

  • use createCriticalStyleStream/getCriticalStyles to inline critical CSS
  • use createStyleStream/getUsedStyles to use .css files
  • use alterProjectStyles with filter options to create two different sets of styles: not yet cache set for critical styles, and the cached ones for used.
  • yes - you have to use or two transformers, or call two functions, one after another.

Theoretically - all styles "critical" now, are "cached" ones next view.


Almost unmeasurable. It's a simple and single RegExp, which is not comparable to the React Render itself.


comparing with tools listed at Google's Optimize CSS Delivery

  • penthouse - a super slow puppetter based solution. No integration with a real run time renderer is possible. Generates one big style block at the beginning of a file.
  • critical - a super slow puppetter based solution. Able to extract critical style "above the fold".
  • inline-critical - slow jsdom based solution. Generates one big style block at the beginning of a file, and replaces all other links by async variants. However, it does not detect any critical or used styles in provided HTML - HTML is used only as a output target. 👎

used-styles is faster that libraries listed above, optimized for multiple runs, as well as




npm i used-styles

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