How to defines XHTML Validation

You have to define correct validation type of each document (Document Type Definitions). Once your document is ready you should validate that on w3c markup validation service site.

HTML 5

<!DOCTYPE html>

This declaration needs to be the first in your HTML document.

<!DOCTYPE html>
<head>
  ...
  ...
</head>
<body>
  ...
  ...
</body>
</html>

Following are other DOCTYPES that are used on older version. Document type definitions is a define the XHTML type of document is Transitional, Strict, and Frameset.

HTML 4.01 Strict

All HTML tag and attributes are allow to supported document type.It's not allow to frame set and presentational.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

HTML 4.01 Frameset

HTML 4.01 Transitional allows to use frameset.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">

HTML 4.01 Transitional

HTML 4.01 Transitional allow to use presentational.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/frameset.dtd">