MathML: spacing after \mathrm containing single letter

Describe the bug: Extra spacing appears after \mathrm{...} containing a single letter [a-zA-Z], for MathML output.

(La)TeX code:

\mathrm{d}x

See JSFiddle.

Expected behavior: No spacing between \mathrm{d} and x.

Screenshot:

Environment (please complete the following information):

• KaTeX Version: 0.11.1
• Device: Desktop
• OS: Debian GNU/Linux 9.12 (stretch)
• Browser: Firefox
• Version: 71.0

Additional context:

• Weirdly, if one wraps either a digit (\mathrm{1}) or more than one character (\mathrm{dd}), the after-spacing does not appear.
• Usually I define a total differential macro like this: \mathop{}\!\mathrm{d}.

[ronkok]

how KaTeX decides what MathML to output

I’m not one of the original KaTeX developers and cannot speak to the thinking behind those decisions. I believe that early KaTeX wrote each character in its own MathML element. More recent versions have done some consolidation for better semantics. For instance, numbers, \text{…}, and \operatorname{…} each consolidate their contents into a single MathML element. Perhaps other consolidations would be beneficial.

[kevinbarabash]

Adding a <mi></mi> to work around what appears to be a browser bug isn’t ideal. A Firefox-only CSS tweak to change the spacing might be better.

Using <mo> &DifferentialD; </mo> for differentials would be awesome. The tricky thing is that the LaTeX for the dx is like any other identifiers. In order to recognize it as a differential operator we’d need to introduce a bunch of additional logic.