The W3C (World Wide Web Consortium) Markup Validation Service lets you check your web documents for markup validity in HTML, XHTML, SMIL, MathML, etc.
To analyze your web documents, go to http://validator.w3.org/ and input the web address of the web document you want to validate. This service will let you know how many markup errors are on your website and how to fix them. If you can fix all the errors on your webpages, your page will be W3C validated and passed.
Validation helps future-proof your website making sure it will work on browsers for years to come. It also eases maintenance and teaches good practices. It is a sign of professionalism. Validation can be used to check whether the code of your website is clean work from a seasoned HTML author.