An embedded HTML page:
<object data="snippet.html" width="500" height="200"></object>
https://www.w3schools.com/tags/tag_object.asp
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"?
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"?