Validation is in fact, the process of checking documents against this formal standard.Converting your HTML web pages to XHTML takes time.One feature I’ve added to the HTML5 validator to mitigate this is a “Message filtering” feature that lets you persistently ignore error/warning messages that aren’t important or useful to you.
Not all document types are supported, because most of them should be deprecated in practice.
CSS is used for style and designing from the internet pages.
W3C or world wide web consortium is a well-known standard which maintains this language.
For large sites, you should download the w3c validator script and install it on your own server.
For Debian based distributions you can get this by installing the w3c_markup_validator package.