Is my page valid? Do I care?

You can use the W3C's validation service at http://validator.w3.org

But is validation that important?

I tried feeding the validator the home pages of the most visited sites the web, with these results:

Facebook redirected the validator to its mobile version, so I've not included it.

So of those sites, only Craigslist passed validation. eBay and Amazon, with over a thousand errors between the two home pages, obviously don't feel that valid HTML should hold them back. Even Google - whose home page has almost nothing on it - has dozens.