Revisit display: contents accessibility once more thorough API is available

[cookiecrook]

Once we have more API available to test (like isIgnored), we should revisit display: contents and do more thorough testing.

Originally posted by @cookiecrook in #60 (comment)

Update from @rahimabdi in web-platform-tests/wpt#43740 (comment)

• Keyboard focusability of display:contents controls (Focusability of elements with display:contents mozilla/standards-positions#772)
• Reading order of accessibility elements such as table cells
• Expected keyboard operability such as controls being focusable, links remaining operable with Enter/Return
• Ensuring elements are exposed appropriately as a result of display: contents (needs isIgnored or similar)
• <table> structure such as accessibility row or column count