Define accessibility and tests for <map> and <layer> polyfills

[prushforth]

Per this comment, have created an issue to track what issues we encounter with polyfilling the <map> element

[prushforth]

@Malvoz

I have updated the demo site so that it will work on Safari, a bit (I don't have routine access to Safari, so hard to test it), by replacing the use of the <map> element with a <mm-app> element (renamed <geo-map> to <mm-app>, because it's not geo-specific, and I seriously hope we are able to get the platform to consider <map> for I think pretty good reasons). I take "transparent content model" to mean it presents content that you put there as though the <map> element didn't actually exist. If that is true, the polyfill content should present a root node, perhaps a div as currently implemented, as though it is a future element. I believe at least a part of accessibility means that the map element should be focusable, so that the user can pan/zoom/query using key strokes appropriately mapped. The controls inside the shadow root should themselves be reachable using tab stops, and each should be completely actionable using key strokes.

[Malvoz]

FYI I filed w3c/html-aam#292.

[prushforth]

Re-open if/when necessary. I would like to not have to discuss the <map> element more than necessary in future.