W3C Validation
The HTML Validation is a developed & announced by the World Wide Web Consortium (W3C) that allows Internet users to check HTML and XHTML documents for well-formed markup.
This technique is specially developed for the web application to be executed in all the browsers (IE, Firefox, Safari, Opera, etc..,) & Operating systems (Windows, Mac, etc..,).
Markup validation is an important technique to measure the quality of web pages.
Browser accommodation:
Many major browsers are often tolerant of certain types of error, and may display a document successfully even if it is not syntactically correct. Certain other XML documents can also be validated if they refer to an internal or external DTD.
W3C also offers validation tools for web technologies other than HTML/XHTML, such as MathML or CSS.
