I believe the sentence from the RFC:
[XHTML1] defines a profile of use of XHTML which is compatible with HTML 4.01
is technically incorrect. While the XHTML 1 compatibility profile was compatible with HTML 4 as implemented by major browsers, that wasn't actually HTML 4. HTML 4 is based on SGML, while what was implemented was a combination of HTML 4 semantics with the tagsoup parsing rules that browsers organically developed. These rules were only later formalized as part of HTML 5.
The compatibility guidelines do recommend a space between <br and />, but (at least according to https://validator.w3.org/ in HTML 4 mode) this doesn't change anything about <br /> being a NET-enabling start-tag <br /, followed by a greather-than sign.
Enter this:
<h1>Hello<br />world</h1>
[H1] Hello>world
Haha yes. Appendix C gave compatibility guidelines, but you are right that doesn’t actually result in documents that could be parsed by a parser that implemented SHORTTAG NET.
Elsewhere in the thread, I posted an example of SHORTTAG NET being removed from a browser to enable parsing of XHTML documents:
https://github.com/emacsmirror/w3/commit/68af7c107dcbe194e30...
Nevertheless, the text/html RFC explicitly condones Appendix C, so despite it not being fully reflective of reality, it’s still permissible to use text/html to label XHTML 1.0 documents that follow Appendix C :D