To identify information as an HTML document conforming to this specification, each document must start with one of the following document type declarations.
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
This document type declaration refers to the HTML DTD in 9.1, "HTML DTD".
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Level 2//EN">
This document type declaration also refers to the HTML DTD which appears in 9.1, "HTML DTD".
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Level 1//EN">
This document type declaration refers to the level 1 HTML DTD in 9.3, "Level 1 HTML DTD". Form elements must not occur in level 1 documents.
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Strict//EN"> <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Strict Level 1//EN">
These two document type declarations refer to the HTML DTD in 9.2, "Strict HTML DTD" and 9.4, "Strict Level 1 HTML DTD". They refer to the more structurally rigid definition of HTML.
HTML user agents may support other document types. In particular, they may support other formal public identifiers, or other document types altogether. They may support an internal declaration subset with supplemental entity, element, and other markup declarations.