Detect non-matching CSS classes

[mthuurne]

A page could contain a CSS class that doesn't exist, for example because of a typo or maybe the project moved to a newer version of Bootstrap and this page wasn't migrated. It would be useful to report such situations, since in the best case the page code or template could be cleaned up and in the worst case it doesn't render correctly.

There could be other reasons why a class doesn't match any CSS rules, for example it is attached to the wrong kind of tag.

Like with #6, we'd need proper CSS parsing to implement this.