What is the new DOCTYPE?

There are several different DOCTYPES for HTML, XHTML, and Framesets and each of these has two modes Strict and Transitional. Strict says that your markup is using the defined standards exactly. See W3C DTDs page for further details.

A document type declaration, or DOCTYPE, is an instruction that associates a particular SGML or XML document (for example, a webpage) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML1.0 - HTML 4.0).

Basically, the DOCTYPE describes the HTML that will be used on your page. A doctype defines which version of HTML/XHTML your document uses. There are several different DOCTYPES for HTML, XHTML, and Framesets and each of these has two modes Strict and Transitional.