Validation is the process of comparing your HTML or CSS to W3C 'Recommendations'. They are called Recommendations rather than Standards because the W3C is not an official standards-making body and members are encouraged, but not obligated, to comply. Most members perceive that it is in their interest to do so, however.
The W3C Recommendations are the primary means by which site developers and browser developers communicate. If both parties agree on what the Recommendations mean and then implement in accordance with them, all web browsers will display all web pages in a consistent manner.
Validation ensures that a web page or style sheet follows the Recommendations. A valid page is more likely to be displayed in multiple browsers as the author intended than is an invalid one.
If you do not validate your pages and correct the errors that the validator reports, you increase the probability that the pages will not be rendered as you intended in various browsers. NOTE: Validation does not ensure that your page is free of defects, only that it conforms to the Recommendations, just as a spell-checker cannot force you to create great literature.
Validate your HTML and CSS. Fix any errors that are reported. Fix, or have a good reason for not doing so, any warnings that are reported.
Last revised 30 Jan 2006