I visited a site a few days ago that had a really nice interface, and (because I'm a nerd) I peeked at the source code.
It was all HTML5. New tags, like <section> and <nav> in there. And lots of schamncy CSS with piles of webkit- and moz- border rounding and gradient layering and great stuff like :first-child selectors.
And I was inspired. Wow, thinks I, I could do this too - the code is lean, the styling is simplified without all the needless background images and workarounds for nice-looking buttons and form UI. Elements aren't overloaded with class names. The markup is beautiful, or at least as beautiful as markup can be. The DOCTYPE isn't strict, it's just "html".
Then I went to another computer and looked at the same site in IE8 on Windows. And it looked like barf. Worse than barf. Like dog barf after the dog has eaten a squirrel.
So, thinks I, it's not time to start redesigning my sites yet.