jonathantneal
Focus Within 
Focus Within lets you style elements when they are focused or contain a focused element, following the Selectors Level 4 specification.
.field label {
/* style a label */
}
.field:focus-within label {
/* style a label when its field also contains a focused element */
}Usage
From the command line, transform CSS files that use :focus-within selectors:
npx focus-within SOURCE.css TRANSFORMED.cssNext, use your transformed CSS with this script:
<link rel="stylesheet" href="TRANSFORMED.css">
<script src="https://unpkg.com/focus-within/browser"></script>
<script>focusWithin(document)</script>That’s it. The script is 379 bytes and works in all browsers, including Internet Explorer 9.
How it works
The PostCSS plugin duplicates rules containing
:focus-within, replacing them with an alternative [focus-within] selector.
.field:focus-within label {
font-weight: bold;
}
/* becomes */
.field[focus-within] label {
font-weight: bold;
}
.field:focus-within label {
font-weight: bold;
}Next, the JavaScript library adds a focus-within
attribute to elements otherwise matching :focus-within natively.
<html focus-within>
<body focus-within>
<form focus-within>
<div class="field" focus-within>
<label for="a">Field</label>
<input id="a" value="This element is focused" focus-within>
</div>
<div class="field">
<label for="b">Field</label>
<input id="b" value="This element is not focused">
</div>
</form>
<p>Some sibling text element.</p>
</body>
</html>GOTCHA!
One cannot simply add the
[focus-within]selector to an existing rule:.field:focus-within label, .field[focus-within] label {}Browsers that don't support
:focus-withinwill throw the entire rule away! This is why you should follow the Usage instructions.