This bug in docbook-xsl-ns version 1.78.1 affects the following outputs: html, xhtml, xhtml1_1, xhtml5
When a Docbook CALS table is created with the attribute rowheader="firstcol", and one of the first column row entries is given the attribute morerows="2" or more, successive non-first column entries are erroneously marked as table headers (i.e. given the tag th) in the HTML output.
Attached is a minimal example and an illustrative screenshot. Notice how in the screenshot "e" and "a" are marked in bold, because in the HTML they are mistakenly given <th>, while the surrounding row entries are non-bold because they are correctly marked up in the HTML as <td>.