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It's also not a tag, but an SGML (?) directive (specifically a https://en.wikipedia.org/wiki/Document_type_definition https://en.wikipedia.org/wiki/Document_typ

It's also not a tag, but an SGML (?) directive (specifically a https://en.wikipedia.org/wiki/Document_type_definition https://en.wikipedia.org/wiki/Document_type_definition) and so it is in fact invalid to produce a closing tag.

vdfs 2y ago
https://stackoverflow.com/q/23005510/3896300 https://stackoverflow.com/q/23005510/3896300
electroly 2y ago
HTML5's HTML syntax is no longer SGML-based. It's still invalid but no longer because of anything related to SGML; now it's simply because the HTML5 spec says so.