Why Validate?

As defined by the W3C:
Validation is a process of checking your documents against a formal Standard, such as those published by the World Wide Web Consortium (W3C) for HTML and XML-derived Web document types, or by the WapForum for WML (Wireless), etc. It serves a similar purpose to spell checking and proofreading for grammar and syntax, but is much more precise and reliable than any of those processes because it is dealing with precisely-specified machine languages, not with nebulously-defined human natural language.

5 Reasons to Validate:

• If you want your site listed on search engines (and who doesn't?) then make sure that you have good HTML. Many engines cannot properly catalog or index a site that has serious HTML errors. This can greatly reduce the amount of traffic your web site receives from search engines.
• Properly written HTML will render better, render on more browsers, and render faster than HTML with errors. It's also more easily adapted to print and alternative browsing devices like mobile phones and handheld computers.
• Properly written HTML is more likely to be "future-proof" (backward compatible with future standards and future web browsers).
• Broken links can quickly drive visitors away. How many times have you been annoyed when you found a broken link?
• Problems such as "ugly" pages caused by poor HTML constantly drive visitors away from web sites. Do you want your web site to be one that customers will leave because of poor quality?

Importantly, in the many industrialized countries with human-rights or disability-discrimination laws, you are legally required to provide accessible Websites. While worldwide legislation for this requirement is still evolving, precedence has occurred (Maguire v SOCOG 2000). Not adhering to this law, fluid as it is, can land you in trouble through possible lawsuits and fines.

Take time to read about the W3C, it's origins, founder, and World Wide Web inventor, Tim Berners-Lee. The W3C site is a deep resource and you will come away impressed with the collective minds steered together "to extend the Web's benefits to all people on the planet."