It is XML, so not all HTML5 is allowed. You would have to convert your HTML5 to XHTML5. Possible but harder than string-concatenation.
It requires full self links, so you must put your domain name in there as relative links would be an error. When using a static generator, why should I care where the files are published?
My website has valid Atom feeds. Still, it's an annoying format to me.
I suspect you've been making your lifeIfe difficult for no good reason.
> why should I care where the files are published?
You don't, but those consuming your feed do. You do it for their benefit.
In most environments it’s no harder, and in browsers themselves (which is where half of it is consumed) it’s arguably easier.
When you parse a document or fragment that is in HTML syntax, the DOM representation is of elements in the http://www.w3.org/1999/xhtml namespace.
There is no difference.