HTML <!DOCTYPE> element

<!DOCTYPE> element specifies HTML version and language of current document by referring to a Document Type Definition (DTD). Browsers display web page content based on the DTD. <!DOCTYPE> must be put at the very beginning of the HTML document.

The <!DOCTYPE> declaration of this page:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 

In HTML5, <!DOCTYPE> is simplified, it do not need a DTD reference, and the HTML5 capable browser can parse the content correctly.

A typical HTML document: