DeclareMathOperator with MathML does not work with styled text

[fatanassow]

Using the MathML writer, this works:

\DeclareMathOperator{\den}{den}

but this:

\DeclareMathOperator{\den}{\mathsf{den}}

produces visible occurrences of \operatorname{\mathsf{den}} all over the text. I thought this might be because the parser for \DeclareMathOperator wrongly expects only a string in the second brackets, but it works as expected with the MathJax writer: the operator gets rendered in sans-serif.

I also tested this in LaTeX to make sure \DeclareMathOperator really accepts something besides simple strings for the operator name, and it does. Here is proof:

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator{\den}{\mathsf{den}}
\begin{document}    
$\den t = \den t'$    
\end{document}