Disable the toolbar button for bold text when editing a header (th) cell of a table. th elements have bold cell text by default, so we don't need to provide an option which is redundant or has bad effects in later appearance.

Well, the problem there is that wikis (and other places VisualEditor can integrate things into) can over-ride the common-to-many browser styles and show these as italic, or underlined, or flashing neon purple. Making assumptions about the visual impact of (a) semantic structural things like <th>, and of (b) semantic inline things like <b>, seems like a bad idea. Theoretically we could build a visual styles comparison engine into VisualEditor to assess whether or not tools would show up viably, but it would be horrendously complex and computationally expensive (== slow for users and bug prone), and it would ignore non-visual semantics for e.g. screen readers.