[TASK] Add Unit Tests for HtmlParser

Make sure the core functionality is unit-tested.

Also, add some more information for some methods.

Resolves: #72664
Releases: master
Change-Id: I3fdfba215955237bc9ec8f19a6a70e2c8142236e
Reviewed-on: https://review.typo3.org/45852


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>