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:
- Google - 35 Errors, 2 warning(s)
- Yahoo - 163 Errors, 33 warning(s)
- Youtube - 100 Errors, 2 warning(s)
- Bing - 12 errors
- Twitter - 20 Errors, 70 warning(s)
- Amazon - 498 Errors, 100 warning(s) !
- eBay - 514 Errors, 24 warning(s) !!
- Flickr - 39 Errors, 9 warning(s)
- Craigslist - successfully checked as HTML 4.01 Transitional!
- IMDB - 215 Errors, 137 warning(s)
- MySpace - 51 Errors, 3 warning(s)
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.