I emailed some folks individually, but I suppose I should also make a note here. At least Chrome, Firefox, and Safari have extended the HTML parser beyond the HTML Standard with the addition of special parsing rules for <rb> and <rtc> elements. (And slightly changed parsing rules for <rp> and <rt> elements.)
Chrome uses HTMLUnknownElement for these elements, Firefox and Safari use HTMLElement. Firefox has also changed styling for these elements. It's unclear to me what Edge has done. >From this it seems pretty clear we should change the parsing rules, for which the first commit of https://github.com/whatwg/html/pull/101 provides my recommended solution. It's less clear to me we should be changing anything else. -- https://annevankesteren.nl/
