ally.query.tabsequence: `<area>` vs `<img usemap>` location in sequence

[rodneyrehm]

Issue with ally.query.tabsequence

While Blink and WebKit add <area> elements to the tabbing sequence where they occur in the DOM, Firefox and IE do not. Gecko and Trident remove <area> from the sequence and inject them (possibly multiple times) for every <img usemap> referencing the <map> those <area>s are defined in.

Trident will redirect focus from <img usemap> to the first <area> of the referenced <map>. Firefox does not do this. As this is undetectable behavior, differention in code would have to rely on platform.js (i.e. user agent sniffing).

This should not require refactoring util/sort-element-by-tabindex, since the issue only exists with ally.query.tabsequence.

[rodneyrehm]

Blink is going to go with the order Gecko and Trident have, leaving WebKit as the odd-man-out for now.