[accessibility] Designating header cells in tables

[Karl Berry]

FWIW, I asked Michal Hoftich (the tex4ht maintainer) about
configuring tex4ht to emit <th> cells.  It's possible, but not a feature
present out of the box.  His reply, for posterity:

Date: Sat, 2 Apr 2022 00:51:20 +0200
From: Michal Hoftich <michal.h21 at gmail.com>

it is possible, you can test for table row using \ifnum\HRow >1
td\else th\fi. The actual configuration would be bit mor ecomplicated,
see \Configure{tabular} in html4.4ht. [...]