Outline naming for contendediable elements

[scottaohara]

From @dd8, originally posted in #160:

What should happen for elements with the contenteditable attribute? Seems that AT has supported aria-label on these for a long time:
https://www.w3.org/WAI/GL/wiki/Using_aria-label_to_provide_an_invisible_label#Example_1:_A_quiz_using_divs_set_to_contentEditable

Also looks like you need to allow aria-label on unexpected places like style elements when contenteditable=true. See the second example using <style contenteditable=true> for live CSS editing in the HTML Living Standard:
https://html.spec.whatwg.org/multipage/interaction.html#attr-contenteditable

(BTW the CSS live editing example works in FF, Chrome and Safari)

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As this is a global attribute, which I've raised an issue about clarifying why, on the HTML spec, there can be some... unexpected behavior with what happens when its used on certain elements.

Beyond that, there may be some scenarios where the attribute may be used on an element that cannot have an accessible name provided.

All that said, we should look at at least providing a note about how accessible names might be generally provided to elements that have contenteditable