FTA
> We’ve got <iframe>, which technically is a pure HTML solution, but
And then on the following paragraph..
> But none of the solutions is HTML
> None of these are a straightforward HTML tag
Not sure what the point is. Maybe just complaining
Iframe is stuck in a rectangular box. It's not really suitable for things like site wide headers, footers and menus.
<iframe> is different from what the author is asking for, it has its own DOM and etc. He wants something like an SSI but client side. He explains some of the problems right after the part you cut off above
"We’ve got <iframe>, which technically is a pure HTML solution, but they are bad for overall performance, accessibility, and generally extremely awkward here"