You must be my doppelganger, because I've been saying exactly this for some time. Internet Explorer caused so much trouble partly because it accepted so much bad HTML. It would close tags for you, even.
I'm not even sure it was really considered a bad thing at the time.
http://www.w3.org/MarkUp/draft-ietf-iiir-html-01.txt
P: Paragraph mark
The empty P element indicates a paragraph break.
The exact rendering of this (indentation,
leading, etc) is not defined here, and may be a
function of other tags, style sheets etc.
<h1>What to do</h1>
This is a one paragraph.< p >This is a second.
< P >
This is a third.
I think IE6 did a lot of awful things, but the only real "damage" done was in borking the CSS implementation (especially the box model). The fact that it rendered crap HTML sort-of-ok wasn't all that bad.
It was the crazy things it did with, lets say.... specially crafted HTML that led to a lot of problems. And MS FrontPage's insisting on adding COLOUR="#000000" (black) to all documents, and assuming BACKGROUND was set to white did a lot to break semantic mark-up.
But I also think it's worth remembering that IE5/6 also featured a lot of early experiments in things that are now considered essential components in the web. XMLHttpRequest was a huge advancement, and while ActiveX was obviously crappy it was at least part of an attempt at creating a more dynamic web.
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.
<a href=foo/bar/baz>qux</a>
<a href="foo">bar</a>baz>qux</a>