I am on a Joomla 5.3 site in the administrator and now look at the Help pages... There are several pages with the weird rendering of the HTML that was reported by @brianteeman in issue #46253 . Same Help pages are served in Joomla 6.

So, my supposition about it being caused by an untranslated page seems not to be (completely) correct. It is very random what pages have this, and what not 😒

Screenshot above was with lang=nl. With lang=en no problem on that "Start Here" page. But for instance https://help.joomla.org/proxy?keyref=Help53:Admin_Modules:_Login_Form&lang=en does give the same weird rendering.