Re: [tex4ht] How to find causes of WARNING] domfilter: DOM parsing of indexsubsection477.htm failed:

[Karl Berry]

    You can find the place where the error happens by looking at the
    byte position in the file specified by the value after char.

I sometimes try to find HTML/XML errors by doing view-source in a
browser on the file. The colorization and different context (i.e., not
the editor) sometimes helps. FWIW ... -k