What was showed in textminus in tex4ht may be good for small changes, but correct process is different. Because
textcomp is basically font selection package, existing
tex4ht support is not done by redefining macros in
textcomp.4ht file, but a special mapping file for font used by
tcrm.htf exists. It is located in
texmf-dist/tex4ht/ht-fonts/unicode/jknappen/tc directory. You can copy this file to your working dir and edit it according to your needs.
.htf files are used by
tex4ht processor to convert glyphs in the
dvi file to internal representation, which is then converted using
.4hf files to actual characters.
.htf files is described in tex4ht help. We need to find position of
\textminus in this file and correct provided definition. It corresponds to position in font encoding.
textcomp package uses
TS1 font encoding, which is defined in ts1enc.def. If we search for
\textminus, we find this line:
This means that
\textminus is defined on position 61 in the
'-' '' 61
as you can see,
- is hardcoded here. We can modify that to use unicode minus instead:
'−' '' 61
It is best to use this hexadecimal form, because it is used by
.4hf files to convert to the actual character.
If you compile your file with
htlatex textput now, and watch output of
tex4ht command, you will see this line:
if you search this file for
x2212, you will find line:
'−' '' '-' ''
this means that unicode minus is converted back to
-. This is why this work only with unicode output with
htlatex texput "xhtml, charset=utf-8" " -cunihtf -utf8"
.4hf file is used now:
different definition is used there:
'−' '' '\226\\136\\146\' ''
this converts entity to the unicode value.
Now back to this question, situation is simpler, as you have added your own symbol at position 61, where original
\textminus is placed and which we already redefined. So this will work also for