Weaken tests for invalid HTML
This is galling because it means it'll be easier to make mistakes in views and templates, but I don't see much that we can reasonably do about it. libxml2 >= 2.14.0 no longer does many of the checks it used to do (https://gitlab.gnome.org/GNOME/libxml2/-/commit/e179f3ec0ef3238ca2e23693cdbc271c7480998f), and HTML 5 validation can't be done with XML DTDs or similar any more.
If we can find a separate validator library then we can slot it in later, but the only things I could find in Python were either designed for other purposes (tidying or sanitization) or involve calling out to a web service.
Fixes: #953 (closed)