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.

Read more about critical style extraction and this library: https://dev.to/thekashey/optimising-css-delivery-57eh

• 🚀 Super Fast - no browser, no jsdom, no runtime transformations
• 💪 API - it's no more than an API - integrates with everything
• 🤝 Works with strings and streams
• ⏳ Helps preloading for the "real" style files

Works in two modes:

• 🚙 inlines style rules required to render given HTML - ideal for the first time visitor
• 🏋️‍♀️inlines style files required to render given HTML - ideal for the second time visitor (and code splitting)

Critical style extraction:

• 🧱 will load all used styles at the beginning of your page in a string mode
• 💉 will interleave HTML and CSS in a stream mode. This is the best experience possible

How it works

1. Scans all .css files, in your build directory, extracting all style rules names.
2. Scans a given html, finding all the classes used.
3. Here there are two options: 3a. Calculate all style rules you need to render a given HTML. 3b. Calculate all the style files you have send to a client.
4. Injects <styles> or <links>
5. After the page load, hoist or removes critical styles replacing them by the "real" ones.

Limitation

For the performance sake used-styles inlines a bit more styles than it should - it inlines everything it would be "not fast" to remove.

• inlines all @keyframe animations
• inlines all html, body and other tag-based selectors (hello css-reset)
• undefined behavior if @layer a,b,c is used multiple times

Speed

Speed, I am speed!

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

API

Discovery API

Use it to scan your dist/build 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 this file from the set, true - add it, and number would change the order of the chunk. Keeping chunk ordered "as expected" is required to preserve style declaration order, which is important for many existing styles.

// with chunk format [chunkhash]_[id] lower ids are potentialy should be defined before higher
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];
});

⚠️ generally speaking - this approach working only unless there are no order-sensive styles from different chunks applied to a single DOM Element. Quite often it never happen, but if you are looking for a better way - follow to #26 ☣️

1. loadStyleDefinitions is a "full control API", and can used to feed used-styles with any custom data, for example providing correct critical css extraction in dev mode (no files written on disk)

return loadStyleDefinitions(
  /*list of files*/ async () => cssFiles,
  /*data loader*/ (file) => fetchTxt(`http://localhost:${process.env.DEV_SERVER_PORT}/${file}`)
  /*filter and order */ // (file) => order.indexOf(cssToChunk[file])
);

Scanners

Use to get used styled from render result or a stream

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

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

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

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

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

React

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 = () => (
  <style data-used-styles dangerouslySetInnerHTML={{ __html: getCriticalRules(markup, styleData) }} />
);

2. React produces more valid code, and you might enable optimistic optimization, making used-styles a bit faster.

import { enableReactOptimization } from 'used-styles';

enableReactOptimization(); // just makes it a but faster

Example

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 random CSS might be not 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, giving you the best 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 express from 'express';
import { 
  discoverProjectStyles, 
  loadStyleDefinitions,
  createCriticalStyleStream, 
  createStyleStream,
  createLink,
} from 'used-styles';

const app = express();

// generate lookup table on server start
const stylesLookup = isProduction
  ? discoverProjectStyles('./dist/client') 
  // load styles for development
  : loadStyleDefinitions(async () => []);

app.use('*', async (req, res) => {
  await stylesLookup;

  try {
    const renderApp = (await import('./dist/server/entry-server.js')).default;

    // create a style steam
    const styledStream = createStyleStream(stylesLookup, (style) => {
      // _return_ link tag, and it will be appended to the stream output
      return createLink(`dist/${style}`) // <link href="dist/mystyle.css />
    });

    // or create critical CSS stream - it will inline all styles
    const styledStream = createCriticalStyleStream(stylesLookup); // <style>.myClass {...

    await renderApp({ res, styledStream });
  } catch (err) {
    res.sendStatus(500);
  }
});

// entry-server.js
import React from 'react';
import { renderToPipeableStream } from 'react-dom/server';
import App from './App';

const ABORT_DELAY = 10000;

async function renderApp({ res, styledStream }) {
  let didError = false;
  
  const { pipe, abort } = renderToPipeableStream(
    <React.StrictMode>
      <App />
    </React.StrictMode>,
    {
      onShellError() {
        res.sendStatus(500);
      },
      onAllReady() {
        res.status(didError ? 500 : 200);
        res.set({ 'Content-Type': 'text/html' });

        // allow client to start loading js bundle
        res.write(`<!DOCTYPE html><html><head><script defer src="client.js"></script></head><body><div id="root">`);

        styledStream.pipe(res, { end: false });
        
        // start by piping react and styled transform stream
        pipe(styledStream);

        styledStream.on('end', () => {
          res.end('</div></body></html>');
        });
      },
      onError(error) {
        didError = true;
        console.error(error);
      }
    },
  );

  setTimeout(() => {
    abort();
  }, ABORT_DELAY);
}

export default renderApp;

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

You have to move injected styles out prior rehydration.

import React from 'react';
import { moveStyles } from 'used-styles/moveStyles';
import ReactDOM from 'react-dom/client';
import App from './App';

// Call before `ReactDOM.hydrateRoot`
moveStyles()

ReactDOM.hydrateRoot(
  document.getElementById('root'),
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

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';

removeStyles();

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 readableString = (string) => {
  const s = new Readable();
  s.push(string);
  s.push(null);
  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
  headerStream.push(createLink(`dist/${style}`));
  // or
  headerStream.push(`<link href="dist/${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
];

MultiStream(streams).pipe(res);

// start by piping react and styled transform stream
htmlStream.pipe(styledStream, { end: false });
htmlStream.on('end', () => {
  // kill header stream on the main stream end
  headerStream.push(null);
  styledStream.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.

Performance

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

Comparison

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. 👎

• critters-webpack-plugun - is the nearest analog of used-styles, build on almost same principles.

used-styles is faster that libraries listed above, and optimized for multiple runs.

License

MIT