Accessibility: `ToggleCompositeField` doesn't follow accessibility best practices

[GuySartorelli]

There are various accessibility problems with the ToggleCompositeField right now:

• Uses an interactable h4 element instead of a button element
• Includes a <a href="#"> element inside the h4 - but since the h4 is tabbable the link is redundant
• Missing a bunch of appropriate aria attributes to indicate to screen reader users what they're focused on and what it does when activated (aria-expanded etc)

Probably makes sense to follow the W3C accordion pattern for this. There's also the W3C Disclosure pattern which is similar.

Related

• Accessibility: jQuery UI accordions use tablist and tabpanel roles incorrectly. silverstripe-admin#1965