DOCTYPE

The Document Type Declaration (DTD), or DOCTYPE indicates the formal definition of a particular version of HTML, SGML, or XML. Web browsers use the < !DOCTYPE > tag to determine the layout mode of the document. You can find a description of DTD and specific properties of the various DOCTYPE tags at the w3schools site or in Wikipedia.

Compatibility Mode

Web browsers render pages differently, depending on the DOCTYPE and a number of other factors. Quirks mode or backwards compatibility is used by some browsers to render a page that is not strictly complying with W3C and IETF standards. Improper tag syntax, using depreciated HTML tags or depreciated tag elements can trigger compatibility mode. To determine browser compatibility for a specific HTML tag click the specific tag in the HTML elements table and scroll to the Browser Compatibility section of the document.