Pasting this into https://mathjax.github.io/MathJax-demos-web/input-mml2svg.html does not produce any output for me. The other links as reported.

In T386585#10609715, @HamidRahkooy wrote:

Pasting this into https://mathjax.github.io/MathJax-demos-web/input-mml2svg.html does not produce any output for me. The other links as reported.

same here

Using mrow within mtext is not allowed in MathML3 (https://www.w3.org/TR/MathML3/appendixa.html#parsing_token.content) nor in MathML core https://www.w3.org/TR/mathml-core/

If one uses <math>\textrm{foobar}</math> this exands to <math class="mwe-math-element" xmlns="http://www.w3.org/1998/Math/MathML"><mrow data-mjx-texclass="ORD"><mstyle displaystyle="true" scriptlevel="0"><mrow data-mjx-texclass="ORD"><mtext>foobar</mtext></mrow></mstyle></mrow></math> in contrast <math>\texttt{foobar}</math> fails with the same error. The code guilty is \MediaWiki\Extension\Math\WikiTexVC\MMLmappings\BaseParsing::hBox

What temml.org seems to be doing is to use mtext and translate each individual letter to another UTF-8 char.

Try

\texttt{foobar}

\textrm{foobar}

on temml.org.

LaTeXML does not use any math specific elements but plain span elements.

Mathoid produces

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block" alttext="{\texttt {foobar}}{\textrm {foobar}}">
  <semantics>
    <mrow>
      <mrow class="MJX-TeXAtom-ORD">
        <mrow class="MJX-TeXAtom-ORD">
          <mtext mathvariant="monospace">foobar</mtext>
        </mrow>
      </mrow>
      <mrow class="MJX-TeXAtom-ORD">
        <mrow class="MJX-TeXAtom-ORD">
          <mtext>foobar</mtext>
        </mrow>
      </mrow>
    </mrow>
    <annotation encoding="application/x-tex">{\texttt {foobar}}{\textrm {foobar}}</annotation>
  </semantics>
</math>

Also fails in Client Side MathJax mode.

This is quite high priority as over 2580 instances in 688 articles on en.wikipedia.

The \textrm syntax is not listed in Help:Formula so we could say that its not a supported syntax. We could probably add some rule so that \textrm is treated as \mathrm or possibly as \text. Either that or we treat it as an error and list it at Category:Articles with Maths Errors.

Change #1148925 had a related patch set uploaded (by Physikerwelt; author: Physikerwelt):

[mediawiki/extensions/Math@master] Fix MathJax textrm rendering

https://gerrit.wikimedia.org/r/1148925

Change #1148925 merged by jenkins-bot:

[mediawiki/extensions/Math@master] Fix MathJax textrm rendering

https://gerrit.wikimedia.org/r/1148925