How important is validation to you?

I've recently been thinking about how important it is that your site has perfectly valid (X)HTML and CSS. I had noticed that some browser-specific CSS3 implementations had caused some invalid code. I was just wondering what everyone else thought.

I usually try to make sure my sites use valid XHTML, mostly because of some SEO benefits, and I like seeing that green bar , but I usually follow it as a guideline, and not as an important rule.

I usually care a bit less about CSS validation. Like I said, there are some CSS3 things that cause some validation errors, but I think that as long as the site is accessible, looks nice, and degrades nicely in older browsers, it's okay.

What do you think?