It's easy to see what's wrong with invalid HTML.
What's wrong with valid HTML?
What's wrong with valid HTML?
Browsers tend to have very little problem rendering invalid markup, and the savings costs associated with invalid markup (bandwidth, time, etc.) are very easily enumerated. So I would pose you this question:
What's wrong with invalid HTML, besides lack of some enigmatic adherence to idealism?
> and the savings costs associated with invalid markup
> (bandwidth, time, etc.) are very easily enumerated.
Say, how would invalid markup help http://kod.as/lab/valid/ be even smaller? Sure, that is pretty meaningless document, but take a look at a source and you will see some stuff that can be applied to normal sites. It's valid: http://validator.nu/?doc=http%3A%2F%2Fkod.as%2Flab%2Fvalid%2... http://validator.w3.org/check?uri=http%3A%2F%2Fkod.as%2Flab%...
You could remove DOCTYPE (which is very short compared to HTML4 or XHTML versions), but then you'd throw your browsers to quirks mode, and that is a pain. The whole purpose of DOCTYPE in HTML5 is to force standards-compliant rendering mode, that's it.
And it only looks that browsers have little problems parsing invalid markup. Making sense of tag soup parsing (and making it consistent) was one of the major tasks for HTML5 people.