The article isn't completely clear; but hr-in-select is standardized [0] - and it definitely works in the Firefox v122.0 I'm browsing with.
[0] https://html.spec.whatwg.org/multipage/form-elements.html#th... (see the "Content model" line)
[0] https://html.spec.whatwg.org/multipage/form-elements.html#th... (see the "Content model" line)
[0] https://www.mozilla.org/en-US/firefox/122.0/releasenotes/#no...