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Provided an xhtml doctype like <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">, why is this tag itself allowed to be non self-closing / non closing at all?
Aka why isn't it: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd" />
It's not a tag (element). It's a declaration and that is the syntax for a declaration (doctype declaration specifically).
Other declarations (
ELEMENT,ENTITY,ATTLIST, etc.) have a similar syntax. The>closes the declaration.