> an infinite loop of includes

We can probably copy the specs for <frameset> and deal with it the same way:

https://www.w3.org/TR/WD-frames-970331#:~:text=Infinite%20Re...

    Any frame that attempts to assign as its SRC a URL used by any of its ancestors is treated as if it has no SRC URL at all (basically a blank frame).

> How to ensure that all other browsers implement it in the same way?

Browsers that don't implement the specs will eventually break:

https://bugzilla.mozilla.org/show_bug.cgi?id=8065