I think talking about the origins of HTML as an SGML document type, that no one actually ever implemented as SGML, which lead to "tagsoup", that ultimately was standardized as HTML5 by the WHATWG; would have been a great addition (oh, and that XML is somehow a simplification of SGML, hence the XHTML effort/attempt before HTML5, that actually started in part because XHTML failed)
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I think talking about the origins of HTML as an SGML document type, that no one actually ever implemented as SGML, which lead to "tagsoup", that ultimately was standardized as HTML5 by the WHATWG; would have been a great addition (oh, and that XML is somehow a simplification of SGML, hence the XHTML effort/attempt before HTML5, that actually started in part because XHTML failed)