Comment 1 for bug 1917371

It's not an error it's a warning. I think these days pretty much all
devices support arbitrary ids, so it can be ignored. Or you can just
have the editor auto-fix it for you if you care.

The numeric ids come from pdftohtml, so this should be fixed there, if
at all. Otherwise its just extra work calibre has to do during
conversion to remap ids, for very dubious benefit.

 status wontfix