Web Standards
What is it?
Web Standards are developed by the World Wide Web Consortium (W3C). They are a list of common practices on which you should develop web pages and web applications. This
Why do I have to do this?
You don't. This is a suggestions. Most pages (with some exeptions) preform better than non-standard pages. This means that they load faster, work in more browsers, and have good code.
Validate you web page.
Note: This does not check the accessibility of your page, only they code itself.