Trigger Quirks mode for index/glossary/search HTML pages
Setting with DOCTYPE the public identifier, but not the system identifier will with 4.01 Transitional trigger the Quirks mode.
To match that also make some tags omit the "/", so the validator is more happy.
The used kdoctools CSS files (as of KF 6.1) seem to require Quirks mode for now, otherwise those CSS files will not be loaded and skipped.
Other generated HTML pages (app manuals from kdoctools, man page) also currently trigger Quirk mode, either by not setting a DOCTYPE or also omitting the system identifier.
For the magic how to trigger Quirks mode, see also https://en.wikipedia.org/wiki/Quirks_mode
24.02 backport candidate