An important part of SEO (Search Engine Optimization) is to make sure that a web site can be viewed correctly by Search Engines and Web Browsers. Validation ensures that a web site does not have any issues and is highly recommended. It’s main functions are to check for missing tags, possible compatibility issues etc.
There are a few different types of validation. Whether a file on a web site is a html, css, rss or xml you can use the W3C web site to check for validation problems on a web site to make sure that the web site will be read and loaded correctly by Search Engines and Web Browsers.