While most Web browsers do an increasingly good job of parsing poorly written HTML, some errors are not always caught gracefully. If your site behaves unpredictably when viewed using different web browsers, you may want to run your site though a validator.
What is Validation?
All web sites — including those generated by WordPress — use common languages (HTML, CSS, Javascript, RSS etc.) to “tell” web browsers what to display and how. In order to ensure that these languages are used and interpreted correctly, a number of organizations (such as the World Wide Web Consortium) develop and maintain standards. These standards define how the languages should be used in a web page and how browsers should interpret them.
One analogy for this would be music scores — a universally recognised method for conveying audio information on paper that has been around for hundreds of years. A piece of music that has been correctly transcribed in this fashion can then be performed on many different instruments. But this only works because everyone uses the same rules when transcribing music. The same applies to web pages (the music score) and web browsers (the instruments). Unless web pages follow the rules, the outcome is likely to be poor when they are viewed in different browsers.
Validating a website is the process of ensuring that the pages on the website conform to the published standards. It ensures that your web pages have the best possible chance of being interpreted correctly across the greatest range of software.
Validating Your Pages & Feeds
The World Wide Web Consortium (W3C) offers a number of free, online, validator tools: