Joel on Software has an absolutely magnificent article explaining the difficult situation Microsoft is in with Internet Explorer 8.
Essentially, he explains how and why the web browser is in a no-win situation. Microsoft can enforce "standards" and allow existing web pages to break if they don't meet the standards (something no browser has done before) or they can continue to be backwards compatible and support all those workarounds people have been putting in for years to make-it-work-for-browser-X.
If they choose the first option, which is their current plan, they'll have a hard time convincing the developers of all those existing websites (if they're around any more) to update their sites to conform to the HTML4 and CSS1 standards amongst others. If not, all those webpages will work, but what's the point of standards if they're not enforced?
It's a longish article, but it's very well written and it provides great insight if you're a web developer.