HTML Validators and HTML Validation
An HTML validator is a program that checks your HTML code to make sure it's written properly. The first thing the validator does is check the doctype of the page you're validating. Next, it analyzes the code in the page to see whether it matches the rules defined by that doctype. The validator will flag mistakes like misspelled tag names or unclosed tags.
The World Wide Web Consortium (W3C) provides a free validator to the public. The W3C is responsible for many of the technologies used on the internet. To use the validator follow this url: http://validator.w3.org. Once you're there you can either copy and paste your code into the validator, point the validator to your URL, or upload your webpage to the W3C site. The validator then analyzes the HTML and generates a report saying whether or not your page is valid. Your errors will appear on the report with a line number indicating which line of your code has an error.