\mbin{...}\kern{...} doesn't stack spacing

[edemaine]

Originally reported by @akalin in #982:

\begin{aligned}
& (a \cdot b) \bmod 257 \\
& (a \cdot b) \mathbin{\mathrm{mod}} 257 \\
& (a \cdot b) \kern1mu\mathbin{\mathrm{mod}}\kern1mu 257
\end{aligned}

produces:

The last row is incorrect (relative to LaTeX). I've figured out the reason, but I'm not sure of the solution:

• We have a CSS rule .katex .mbin+.mord { margin-left: .22222em; } that should give the object after an mbin a space before it of .22222em.
• But the \kern1mu makes an element with class mord rule with a manual style of margin-left: 0.0555556em;.
• So instead of the spaces adding up as they do in LaTeX, the second one overrides the first one. ☹️

Thoughts on how to fix this?