HTML doctype

HTML <!doctype> refer to the document type declaration use to specifying which version of HTML the document is using.
HTML <!doctype> declaration is not an HTML tag. It's a information about which version of the markup language the page is written.
Following doctypes are recommended in W3C specification.

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.

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 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 allow to use presentational.

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