I may be wrong here, but isn't implicit tag closing consequence of html deriving from sgml? mandatory tag closing is something that came with xml AFAIK
<a href=foo/bar/baz>qux</a>
as <a href="foo">bar</a>baz>qux</a>
since it did not only include optional closing tags, but also short tags. (This is incidentally why several SGML-based HTML validators used to complain about "end tag for element A which is not open" in the presence of href attributes without quotes.)