Enable "MathML (experimental; no images)" in Special:Preferences → Apperance → Math. Then preview a page with the contents <math>\mathbf{\Lambda}</math>. This should renders as bold Λ but it's actually just an ordinary Λ (lowercase Greek letters are not affected by this). Firefox correctly displays it as bold when you change mathvariant="normal" to mathvariant="bold" in the generated MathML.
I took a look at the MediaWiki code and found two possible issues:
- The function getFontArgs in MMLmappings/Util/MMLParsingUtil.php gets called with $name = \mathbf but compares it against case "mathbf" without the leading backslash.
- The function checkAndParseMathCharacter in MMLmappings/BaseMethods.php changes "mathvariant" back to "normal" anyway.
