Note that the regex and the way the parser considers the commas changed in 16ed15ff0d7c (it now considers them as the name of a value-less attribute), so adding a group for the comma is no longer doable.

In theory, the approach you suggest might work, but if we want some warning mechanism it should be generic enough to work with all kind of invalid markup.  In addition this adds complexity to already complex regular expressions, so there should be a valid use case for this.
Also keep in mind that HTMLParser won't do any check about the validity of the elements' names or attributes' names/values, or even if they are nested/closed correctly, so even with a comprehensive set of warnings, you won't still be able to use HTMLParser to validate your pages.