PostCSS Has Pseudo

npm install css-has-pseudo --save-dev

PostCSS Has Pseudo lets you style elements relative to other elements in CSS, following the Selectors Level 4 specification.

To use this feature you need to do two things :

• add the PostCSS plugin that transforms the selector into a class or attribute
• add the browser polyfill that sets the attribute or class on elements in a browser

.title:has(+ p) {
	margin-bottom: 1.5rem;
}

/* becomes */

.js-has-pseudo [csstools-has-1a-38-2x-38-30-2t-1m-2w-2p-37-14-17-w-34-15]:not(does-not-exist) {
	margin-bottom: 1.5rem;
}
.title:has(+ p) {
	margin-bottom: 1.5rem;
}

Usage

Add PostCSS Has Pseudo to your project:

npm install postcss css-has-pseudo --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const postcssHasPseudo = require('css-has-pseudo');

postcss([
	postcssHasPseudo(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);

Options

preserve

The preserve option determines whether the original notation is preserved. By default the original rules are preserved.

postcssHasPseudo({ preserve: false })

.title:has(+ p) {
	margin-bottom: 1.5rem;
}

/* becomes */

.js-has-pseudo [csstools-has-1a-38-2x-38-30-2t-1m-2w-2p-37-14-17-w-34-15]:not(does-not-exist) {
	margin-bottom: 1.5rem;
}

specificityMatchingName

The specificityMatchingName option allows you to change the selector that is used to adjust specificity. The default value is does-not-exist. If this is an actual class, id or tag name in your code, you will need to set a different option here.

See how :not is used to modify specificity.

postcssHasPseudo({ specificityMatchingName: 'something-random' })

specificity 1, 2, 0

Before :

.x:has(> #a:hover) {
	order: 11;
}

After :

specificity 1, 2, 0

[csstools-has-1a-3c-1m-2w-2p-37-14-1q-w-z-2p-1m-2w-33-3a-2t-36-15]:not(#does-not-exist):not(.does-not-exist) {
	order: 11;
}

⚠️ Known shortcomings

Performance

Determining which elements match a :has selector is relatively slow through a polyfill compared to the native feature.

A very large DOM or many and complex :has selectors can cause performance issues.
JavaScript frameworks that rewrite the DOM will be particularly affected by this.

Any contributions to speedup matching are welcome.
Please open an issue to discuss proposed changes if you are interested in contributing.

Specificity

:has transforms will result in at least one attribute selector with specificity 0, 1, 0. If your selector only has tags we won't be able to match the original specificity.

Before :

specificity 0, 0, 2

figure:has(> img)

After :

specificity 0, 1, 2

[csstools-has-2u-2x-2v-39-36-2t-1m-2w-2p-37-14-1q-w-2x-31-2v-15]:not(does-not-exist):not(does-not-exist)

Plugin order

As selectors are encoded, this plugin (or postcss-preset-env) must be run after any other plugin that transforms selectors.

If other plugins are used, you need to place these in your config before postcss-preset-env or css-has-pseudo.

Please let us know if you have issues with plugins that transform selectors. Then we can investigate and maybe fix these.

Browser

// initialize cssHasPseudo
import cssHasPseudo from 'css-has-pseudo/browser';
cssHasPseudo(document);

or

<!-- When using a CDN url you will have to manually update the version number -->
<script src="https://unpkg.com/[email protected]/dist/browser-global.js"></script>
<script>cssHasPseudo(document)</script>

[!TIP] Please use a versioned url, like this : https://unpkg.com/[email protected]/dist/browser-global.js Without the version, you might unexpectedly get a new major version of the library with breaking changes.

PostCSS Has Pseudo works in all major browsers, including Internet Explorer 11. With a Mutation Observer polyfill, the script will work down to Internet Explorer 9.

Browser Usage

hover

The hover option determines if :hover pseudo-class should be tracked. This is disabled by default because it is an expensive operation.

cssHasPseudo(document, { hover: true });

observedAttributes

The observedAttributes option determines which html attributes are observed. If you do any client side modification of non-standard attributes and use these in combination with :has() you should add these here.

cssHasPseudo(document, { observedAttributes: ['something-not-standard'] });

forcePolyfill

The forcePolyfill option determines if the polyfill is used even when the browser has native support. This is needed when you set preserve: false in the PostCSS plugin config.

cssHasPseudo(document, { forcePolyfill: true });

debug

The debug option determines if errors are emitted to the console in browser. By default the polyfill will not emit errors or warnings.

cssHasPseudo(document, { debug: true });

Browser Dependencies

Web API's:

• MutationObserver
• requestAnimationFrame
• querySelectorAll with support for post CSS 2.1 selectors and :scope selectors.

ECMA Script:

• Array.prototype.filter
• Array.prototype.forEach
• Array.prototype.indexOf
• Array.prototype.join
• Array.prototype.map
• Array.prototype.splice
• RegExp.prototype.exec
• String.prototype.match
• String.prototype.replace
• String.prototype.split

CORS

[!IMPORTANT] Applies to you if you load CSS from a different domain than the page.

In this case the CSS is treated as untrusted and will not be made available to the JavaScript polyfill. The polyfill will not work without applying the correct configuration for CORS.

Example :

| page | css | CORS applies | | --- | --- | --- | | https://example.com/ | https://example.com/style.css | no | | https://example.com/ | https://other.com/style.css | yes |

You might see one of these error messages :

Chrome :

DOMException: Failed to read the 'cssRules' property from 'CSSStyleSheet': Cannot access rules

Safari :

SecurityError: Not allowed to access cross-origin stylesheet

Firefox :

DOMException: CSSStyleSheet.cssRules getter: Not allowed to access cross-origin stylesheet

To resolve CORS errors you need to take two steps :

• add an HTTP header Access-Control-Allow-Origin: <your-value> when serving your CSS file.
• add crossorigin="anonymous" to the <link rel="stylesheet"> tag for your CSS file.

In a node server setting the HTTP header might look like this :

// http://localhost:8080 is the domain of your page!
res.setHeader('Access-Control-Allow-Origin', 'https://example.com');

You can also configure a wildcard but please be aware that this might be a security risk. It is better to only set the header for the domain you want to allow and only on the responses you want to allow.

HTML might look like this :

<link rel="stylesheet" href="https://example.com/styles.css" crossorigin="anonymous">

Using with Next.js

Given that Next.js imports packages both on the browser and on the server, you need to make sure that the package is only imported on the browser.

As outlined in the Next.js documentation, you need to load the package with a dynamic import:

useEffect(async () => {
	const cssHasPseudo = (await import('css-has-pseudo/browser')).default;
	cssHasPseudo(document);
}, []);

We recommend you load the polyfill as high up on your Next application as possible, such as your pages/_app.ts file.

How it works

The PostCSS Has Pseudo clones rules containing :has(), replacing them with an alternative [csstools-has-] selector.

.title:has(+ p) {
	margin-bottom: 1.5rem;
}

/* becomes */

.js-has-pseudo [csstools-has-1a-38-2x-38-30-2t-1m-2w-2p-37-14-17-w-34-15]:not(does-not-exist) {
	margin-bottom: 1.5rem;
}
.title:has(+ p) {
	margin-bottom: 1.5rem;
}

Next, the browser script adds a [:has] attribute to elements otherwise matching :has natively.

<div class="title" [csstools-has-1a-38-2x-38-30-2t-1m-2w-2p-37-14-17-w-34-15]>
	<h1>A title block</h1>
	<p>With an extra paragraph</p>
</div>