PostCSS Focus Within

npm install postcss-focus-within --save-dev

PostCSS Focus Within lets you use the :focus-within pseudo-class 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

.my-form-field:focus-within label {
    background-color: yellow;
}

/* becomes */

.my-form-field[focus-within].js-focus-within label, .js-focus-within .my-form-field[focus-within] label {
    background-color: yellow;
}
.my-form-field:focus-within label {
    background-color: yellow;
}

PostCSS Focus Within duplicates rules using the :focus-within pseudo-class with a [focus-within] attribute selector, the same selector used by the focus-within polyfill. This replacement selector can be changed using the replaceWith option. Also, the preservation of the original :focus-within rule can be disabled using the preserve option.

Usage

Add PostCSS Focus Within to your project:

npm install postcss postcss-focus-within --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const postcssFocusWithin = require('postcss-focus-within');

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

Options

preserve

The preserve option determines whether the original notation is preserved. By default, it is preserved.

postcssFocusWithin({ preserve: false })

.my-form-field:focus-within label {
    background-color: yellow;
}

/* becomes */

.my-form-field[focus-within].js-focus-within label, .js-focus-within .my-form-field[focus-within] label {
    background-color: yellow;
}

replaceWith

The replaceWith option defines the selector to replace :focus-within. By default, the replacement selector is [focus-within]. Please note that using a class, leverages classList under the hood which might not be supported on some old browsers such as IE9, so you may need to polyfill classList in those cases.

postcssFocusWithin({ replaceWith: '.focus-within' });

.my-form-field:focus-within label {
    background-color: yellow;
}

/* becomes */

.my-form-field.focus-within.js-focus-within label, .js-focus-within .my-form-field.focus-within label {
    background-color: yellow;
}
.my-form-field:focus-within label {
    background-color: yellow;
}

Note that changing this option implies that it needs to be passed to the browser polyfill as well.

disablePolyfillReadyClass

The disablePolyfillReadyClass option determines if selectors are prefixed with an indicator class. This class is only set on your document if the polyfill loads and is needed.

By default this option is false. Set this to true to prevent the class from being added.

postcssFocusWithin({ disablePolyfillReadyClass: true })

.my-form-field:focus-within label {
    background-color: yellow;
}

/* becomes */

.my-form-field[focus-within] label {
    background-color: yellow;
}
.my-form-field:focus-within label {
    background-color: yellow;
}

Browser

import focusWithinInit from 'postcss-focus-within/browser';

focusWithinInit();

or

<!-- When using a CDN url you will have to manually update the version number -->
<script src="https://unpkg.com/postcss-focus-within@9.0.1/dist/browser-global.js"></script>
<script>focusWithinInit()</script>

PostCSS Focus Within works in all major browsers, including Safari 6+ and Internet Explorer 9+ without any additional polyfills.

Browser Usage

force

The force option determines whether the library runs even if the browser supports the selector or not. By default, it won't run if the browser does support the selector.

focusWithinInit({ force: true });

replaceWith

Similar to the option for the PostCSS Plugin, replaceWith determines the attribute or class to apply to an element when it's considered to be :focus-within.

focusWithinInit({ replaceWith: '.focus-within });

This option should be used if it was changed at PostCSS configuration level.

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 focusWithinInit = (await import('postcss-focus-within/browser')).default;
    focusWithinInit();
}, []);