committed to tl (r45261), sorry for the delay.

html-mml.4ht giver error:

! Undefined control sequence.
\a:putHBorder ->\bgroup \ifmathml \mathml:hborder
\fi \HCode {\HBorder }\egr...
l.5 a
bc \\[1ex]

I have very little experience in (tex) programming so cannot figure out where the problem is.

mwe.tex:
---
\documentclass{article}

\usepackage{tabularx}%If this is commented out then everything works as expected

\begin{document}
\[\begin{array}{c}
abc \\[1ex]
def
\end{array}\]
\end{document}
---

mwe.cfg:
---
\Preamble{xhtml,mathml-,html5}
\begin{document}
\EndPreamble
---

Compilation with:

$ make4ht -u -c mwe.cfg mwe.tex

---

What works: The example above works as expected if tabularx package is not loaded. The line change inside array produces html that looks like this:

<mtr class="vspace" style="font-size:4.30554pt"><mtd></mtd></mtr>

The problem: When tabularx is loaded the line produced by line change inside array produces to this:

<tr class="vspace" style="font-size:4.30554pt"><td>&nbsp;</td></tr>

The mtr and mtd tags have changed to tr and td which are not a valid mathml tags. It produces error when rendered with MahtJax.