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;
}
.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()
]).process(YOUR_CSS );
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;
}
.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
import cssHasPseudo from 'css-has-pseudo/browser';
cssHasPseudo(document);
or
<script src="https://unpkg.com/css-has-pseudo@7.0.2/dist/browser-global.js"></script>
<script>cssHasPseudo(document)</script>
[!TIP]
Please use a versioned url, like this : https://unpkg.com/css-has-pseudo@7.0.2/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:
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 :
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 :
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;
}
.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>