Even though we have a HTML5 client, we use HTML4 in several places. This includes tlclient.cgi as well as the web administration interface. We should standardize on HTML5 wherever we use HTML.
It's not worth the effort combing through the entire specification for what has changed from HTML5. But we can use the validator and see what it points out on our pages:
All our content should now be fixed. There is one more set of pages that could be HTML 4 though; the error pages generated by Python's standard library. It got very recently changed to use HTML 5:
RHEL 9 is on Python 3.9, so not even that has this change.
Still, the old pages don't use anything deprecated in HTML 5, so probably not worth the effort to override things just to get a newer doctype. Better to look forward to bug 3496 instead.
Tested using server dev build #2971 on Fedora37 in Firefox. Pages evaluated for html5 inconsistencies include all the tlwebaccess and tlwebadm pages (using the html5 validator https://validator.w3.org/).
[✓] All HTML pages should identify themselves as HTML5
[✓] There should be nothing used that HTML5 deems obsolete
Looks like everything is in order. Closing.