DTD or Document Type Declaration

XHTML is stricter in rule implementation as compared to HTML but being in transition from HTML, XHTML gives three levels of compliance to a web page which are indicated by Doctype or Document Type Declaration and which are

Transitional XHTML 1.0 to allow use of outdated markups of HTML 4.1

Strict XHTML 1.0 is used with no outdated markup. It follows XHTML stricter syntax.

Frameset XHTML 1.0 to allow frames in web pages.

Usually transitional XHTML 1.0 is used and the doctype declaration is before <html> tag, and after optional XML Declaration. Different doctype declarations are

For transitional –

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

For strict –

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

For frameset –

<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Frameset//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd”>