Consider adding element subtree to th accessible name computation

[extra808]

Currently the name computation for tr, td, and th elements are the same. I don't have an opinion about the others but for th elements with the columnheader or rowheader role, shouldn't "Otherwise use th element subtree." be included, before "Otherwise use title attribute."?

https://w3c.github.io/html-aam/#tr-td-th-elements-accessible-name-computation

In practice, when a th columnheader contains text and has a title attribute, Chrome (v112) and Firefox (v111) do use the subtree for the name, according to their DevTools. However Safari's (v16.4.1 for Mac) DevTools show the cell's name as being the title attribute's value. Safari's name seems wrong but it's conforming to these mappings as currently written.

[scottaohara]

added this comment to related issue #179 so this one can be closed.