How to Apply HTML 5 Validation:

The <!DOCTYPE> declaration is not an HTML tag. It’s an instruction to the web browser what HTML version the page should be written in.

<!DOCTYPE html> goes at the top of every HTML5 page.

<!DOCTYPE html> means “this page is written in HTML5” as opposed to, say HTML 4.01. Here’s an example of one common type of XHTML:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

XHTML template structure:

<!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">

 

<head>

        <title>An XHTML 1.0 Strict standard template</title>

        <meta http-equiv="content-type"

        content="text/html;charset=utf-8" />

</head>

<body>

     <p>… Your HTML content here …</p>

</body>

</html>

Transitional:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"

   "http://www.w3.org/TR/html4/loose.dtd">

Frameset:

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

XHTML 1.0 Strict:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

Tip: Don’t forget to add the <!DOCTYPE> declaration to your HTML documents, so that the browser knows what type of document to expect. Otherwise you can face some problem with your website.