> "Includes" functionality is considered to be server-side

Exactly! Include makes perfect sense on server-side.

But client-side include means that the client should be able to modify original DOM at unknown moment of time. Options are

1. at HTML parse time (before even DOM is generated). This requires synchronous request to server for the inclusion. Not desirable.

2. after DOM creation: <include src=""> (or whatever) needs to appear in the DOM, chunk loaded asynchronously and then the <include> DOM element(sic!) needs to be replaced(or how?) by external fragment. This disables any existing DOM structure validation mechanism.

Having said that...

I've implemented <include> in my Sciter engine using strategy #1. It works there as HTML in Sciter usually comes from local app resources / file system where price of issuing additional "get chunk" request is negligible.

See: https://docs.sciter.com/docs/HTML/html-include