I'm not so sure they would stay in HTML4. Currently HTTPS is being "forced" by having features that are only available over HTTPS (such as, but not limited to,

I'm not so sure they would stay in HTML4. Currently HTTPS is being "forced" by having features that are only available over HTTPS (such as, but not limited to, HTTP/2 and HTTP/3) and this has been fairly helpful. XHTML adoption could have been encouraged using similar tactics, and most developers would have gone along.