Salvete, I know the MathML spec is inconsistent. I believe the following should be changed, though: <mfenced><mi>x</mi><mi>y</mi></mfenced> currently renders as $(xy)$, while the spec says (3.3.8.1) it should be $(x,y)$. Fix: xtag-mmp.tex, line 105, change from \def\MMLpFENCED#1#2% {\getXMLarguments{mfenced}{open="(" close=")" separators="" #1}% to \def\MMLpFENCED#1#2% {\getXMLarguments{mfenced}{open="(" close=")" separators="," #1}% <mo>⁡</mo> and its equivalent forms should not produce output. <mfenced open=""><mi>x</mi></mfenced> leads to an error, since the \left. at the beginning is missing. (Unlikely to appear in everyday maths, but can happen.) In my reading of 3.2.3.2, <mi>a</mi> should be italic, but <mi>abc</mi> should be upright. (I would be happy if I had to switch this on at only one place, to keep backward compatibility.) regards, Christopher Creutzig