Validation is the process of comparing your code against known standards. Helping you to find and fix errors that may make your site display in unexpected ways in different browsers.
Every day we come across questions on the CSS Forum that are fixed thanks to the magic of validation. An error in your stylesheet may cause the rules after to be avoided. An unclosed tag in your markup can put the page structure out of wack. A simple typo is all it takes.
The best advice that can be given to someone starting out in web development is to validate your site. Get in the habit of validating before you ask for help and maybe you won't need help.
You can validate your markup (HTML, X-HTML, HTML5) and your stylesheets (CSS) and feeds (Atom, RSS).
- W3C Markup Validation Service
- W3C CSS Validation Service
- W3C Feed Validation Service
- W3C Quality Assurance Tools