PostCSS Focus Within [][PostCSS]

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

``css
.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:

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

Use it as a [PostCSS] plugin:

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

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

Options

$3

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

`js
postcssFocusWithin({ preserve: false })
`

`css
.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;
}
`

$3

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.

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

`css
.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.

$3

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.

`js
postcssFocusWithin({ disablePolyfillReadyClass: true })
`

`css
.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

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

focusWithinInit();
`

or

`html



`

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

$3

#### 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.

`js
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.

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

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

$3

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:

`jsx
useEffect(async () => {
const focusWithinInit = (await import('postcss-focus-within/browser')).default;
focusWithinInit();
}, []);
``

[cli-url]: https://github.com/csstools/postcss-plugins/actions/workflows/test.yml?query=workflow/test
[css-url]: https://cssdb.org/#focus-within-pseudo-class
[discord]: https://discord.gg/bUadyRwkJS
[npm-url]: https://www.npmjs.com/package/postcss-focus-within

[PostCSS]: https://github.com/postcss/postcss
[PostCSS Focus Within]: https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-focus-within
[Selectors Level 4 specification]: https://www.w3.org/TR/selectors-4/#the-focus-within-pseudo