mjx-container is role="presentation" but focusable

[pkra]

The combination of role=presentation and tabindex=0 leads to https://w3c.github.io/aria/#conflict_resolution_presentation_none

f an element is focusable, or otherwise interactive, user agents MUST ignore the presentation role and expose the element with its implicit role, in order to ensure that the element is operable.

This is usually flagged by automated checkers.

It's not a real issue since custom elements have no implicit role worth disabling but for the same reason I don't see why it's added either.

[dpvc]

I think we can probably remove the role entirely. This was probably just copied from v2 output that didn't use the custom elements.