Y2K was a specific class of bugs, which was easy to define and focus on. I'm afraid "things that are there for backwards-compat and have been deprecated by standards" is quite a bit more vague.
I love the implicit assumption that the standards-of-the-moment are any better. The only useful current HTML standard is "does it render in webkit." I can't even count the number of websites that can't render at all on devices even a year or two old (looking at you, Forbes).<p>This whole 'effort' is moot since web practice veers so quickly that we'll be back in the same boat by 2020.
Is it really worth breaking all legacy websites?<p>The most important thing to do is to ensure that people use new solutions.<p>Wouldn't it be nice if there was an HTML5 only mode in your browser and it would ignore legacy tags?