Add a focus-within polyfill to core

Problem/Motivation

The CSS :focus-within pseudo-class looks very handy, but we don't use it in core yet. It's particularly useful as a way to achieve better parity between mouse and keyboard interaction, by pairing :hover with :focus-within styles for container elements.

It has good support, but will need a polyfill for some of the browsers we support, in particular Internet Explorer (the forthcoming Chromium-based Edge supports it).

• Several polyfills are available. Most of these work by programmatically adding an atttribute to all ancestors of the element which matches :focus.
• Stylesheets need extra selectors to target the polyfill's attribute, but there are utilities for SASS/Post-CSS to manage this.
• Modernizr has a test for :focus-within support, since version 3.6.0. Currently D8 has version 3.3.1. D9 has modernizer 3.8.0 - #3100937: Update modernizr to 3.8.0.

Examples where :focus-within could help

1. Highlighting table rows. The Seven theme already has this style rule in tables.css, and Claro has something similar:
   

   tbody tr:hover,
   tbody tr:focus {
     background: #f7fcff;
   }

   Mouse users get a highlight on the row currently being hovered over, which is a useful aid to reading, comprehension, and hitting the correct checkboxes and drop-buttons. The tr:focus is intended to provide an equivalent effect for keyboard users too; but it fails because table rows aren't normally focusable, so the selector never matches. What's really needed here is tr:focus-within.

2. Media library truncates long names, but expands them when a keyboard user tabs to a media item, so they can read the whole name. Currently JS events are used to manage a class on a container element, but this could be simplified by a :focus-within approach.
   

           // Adds is-focus class to the click-to-select element.
           .on('focus blur', ({ currentTarget, type }) => {
             $(currentTarget)
               .closest('.js-click-to-select')
               .toggleClass('is-focus', type === 'focus');
           });
   

   .media-library-item--grid.is-focus .media-library-item__name { white-space: normal; }
   

3. Highlighting the label and description associated with a form input. For example a coloured background on div.form-item:focus-within. (The input itself would still use a :focus style.)
4. Highlighting a group of related controls, e.g. date/time, shipping address. Use :focus to style the current control, and style the containing fieldset with :focus-within

Proposed resolution

• Add a focus-within polyfill to core, make it available as a core library which CSS libraries can depend on.
• Optionally load it via modernizr?
• Follow-up issue: change tr:focus to tr:focus-within in Seven's table style.
• Follow-up issue: change tr:focus to tr:focus-within in Claro's table style.

Remaining tasks

Decide which focus-within polyfill to use. What criteria to evaluate this?
Change record.