Page: http://validator.w3.org/
How does it work?
Lets get started, shall we?The Validator is based on OpenSP, an SGML parser based on James Clark's SP SGML parser. The Validator itself is a CGI script that (basically) fetches your ****, passes it through the parser, and post-processes the resulting error list for easier reading.
Now you have to choose which of the options you want the check to be in.
1. You write your address to your site, very useful if you haven't acces to the file.
2. Upload the file from your computer, useful when you have the file on your computer.
3. Upload the source code of your page, useful when you are making the site, just go there and test.
Hit check when you're done!
Now you'll see how many errors you got on your page and you'll see how you can fix them.
It's very useful when you got or think you got errors, if you have any questions or you need help, read the "Help & FAQ" topic.
Remember, this do not harm your site or code in any way! It just help you with your errors.