\textbf{\textit{x}} should produce a bold-italic "x", but it currently produces a nonbold italic "x" because all KaTeX commands reset the entire font. My understanding is that this is correct for math font commands (\mathbf{\mathit{x}} correctly produces a nonbold italic "x"), but incorrect for text font commands. (How asymmetric...) By contrast, \text{\textbf{\it x}} (using \it implemented in #675) should produce a nonbold italic "x", because old font commands like \it should reset the font.

This would be a moot point if there weren't bold italic fonts available, but there are. This would also require creating Main-BoldItalic fonts; see below.