Validation in HTML

What is validation ?
According to the W3c, 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, or by the WapForum for WML, etc.



Specify a DOCTYPE and namespace
Include a DOCTYPE at the top of your page and specify the namespace

For XHTML1.0 Strict


!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
html xmlns="http://www.w3.org/1999/xhtml"


For XHTML1.0 Transitional


!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
html xmlns="http://www.w3.org/1999/xhtml"


For XHTML1.0 Frameset


!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
html xmlns="http://www.w3.org/1999/xhtml"