Basically, 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. 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.

It is important to note that validation has a very precise meaning. Unfortunately the issue is confused by the fact that some products falsely claim to "validate", while in reality applying an arbitrary selection of tests that are not derived from any standard. Such tools may be genuinely useful, but should be used alongside true validation, not in place of it.

We feel it is important that you emphasize standards compliance. By complying with existing standards, rather than relying upon browser specific extensions and hacks, you can make sure that the web sites you design will be readable by all browsers supporting those standards, not just the ones you have time to test it on, and that your page designs won't break when new browsers and versions come into existence. HTML tags that go through the standards process are evaluated more thoroughly and designed for graceful degradation on older browsers.