The automatic tag closing behaviour is even specified now.
But look at it another way: imagine browsers accepted only valid HTML and every browser had some bugs in what they consider valid. Writing a document in a way that it displays at all might be a challenge already then. Add to that that not everyone is on the latest browser version (that was probably more true in the 90s as well) and you get all the current pains just with worse symptoms. I'm not really sure that's better.
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
Yes, but then nobody implemented the SGML definition of HTML correctly. According to the standard, browsers should have parsed
<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.)
But look at it another way: imagine browsers accepted only valid HTML and every browser had some bugs in what they consider valid. Writing a document in a way that it displays at all might be a challenge already then. Add to that that not everyone is on the latest browser version (that was probably more true in the 90s as well) and you get all the current pains just with worse symptoms. I'm not really sure that's better.