Lots of rationalization in here—it's always been needed. I complained about the lack of <include src="..."> when building my first site in '94/95, with simpletext and/or notepad!

It was not in the early spec, and seems someone powerful wouldn't allow it in later. So everyone else made work arounds, in any way they could. Resulting in the need being lessened quite a bit.

My current best workaround is the <object data=".."> tag, which has a few better defaults than iframe. If you put a link to the same stylesheet in the include file it will match pretty well. Size with width=100%, though with height you'll need to eyeball or use javascript.

Or, Javascript can also hoist the elements to the document level if you really need. Sample code at this site: https://www.filamentgroup.com/lab/html-includes/