Allow gridcell and treeitem roles on button elements

[smhigley]

In the Rules of ARIA attribute usage by HTML element , <button> already allows every child role of composite widgets that manage focus except for tree and grid.

There are benefits in using <button> as a base of interaction in these widgets, such as:

• Enter/space triggers the click event
• Certain colors and interaction styles are applied automatically in forced color modes like Windows High Contrast Mode.

Those benefits also apply to gridcells and treeitems, so it seems like it would be valuable to add those as well.

Happy to do a PR for this :)

[scottaohara]

Closing. Reasoning provided in the closed PR #299

[smhigley]

@scottaohara would you mind reopening, since we discussed taking a look at this again?

[scottaohara]

re: discussion mentioned above - the reason this was re-opened is because while HTML's content model will still disallow nested interactive elements within a button element (regardless if its role was overwritten or not), in further testing there does not appear to be any negative impact from a UX pov. Additionally, even in situations where using a base button element for these roles, or if one were to include nested interactive elements - any problems found would either be issues for all users (e.g., pressing a gridcell submits a form unexpectedly) or would likely be found via other wcag related testing (automated or manual).