This has always worked for me. Pretty much the ask?
https://gist.github.com/sreekotay/08f9dfcd7553abb8f1bb17375d...
An embedded HTML page:
<object data="snippet.html" width="500" height="200"></object>
That said - it does appear encapsulated, much like an iframe - css doesn't take, etc. so though more "pure" HTML I'd argue doesn't fulfill the spirit of "include"?
[edit: I'm sure there are still some file:// workflows for docs - and yes this doesn't address that]
:)