> We’ve got <iframe>, which technically is a pure HTML solution, but they are bad for overall performance, accessibility, and generally extremely awkward here
What does this mean? This is a pure HTML solution, not just "technically" but in reality. (And before iframe there were frames and frameset). Just because the author doesn't like them don't make them non-existent.
An iframe is a window into another webpage, and is bounded as such both visually and in terms of DOM interfaces. A simple example would be that an iframe header can't have drop-down menus that overlap content from the page hosting it.
They are categorically not the same DX/UX as SSI et al. and it's absolutely bizarre to me that there's so many comments making this complaint.
The real problem with iframes is that their size is set by the parent document only.
They would be a lot more useful if we could write e.g. <iframe src=abc.html height=auto width=100> so the height of the iframe element is set by the abc.html document instead of the parent document.
What does this mean? This is a pure HTML solution, not just "technically" but in reality. (And before iframe there were frames and frameset). Just because the author doesn't like them don't make them non-existent.