2. HTML Specification

2.5 Understanding HTML and SGML

HTML is an application of ISO Standard 8879:1986 -- Standard Generalized Markup Language (SGML). SGML is a system for defining structured document types, and markup languages to represent instances of those document types. The SGML declaration for HTML is given in Section 5.1. It is implicit among HTML user agents.

If the HTML specification and SGML standard conflict, the SGML standard is definitive.

Every SGML document has three parts:

SGML declaration

Binds SGML processing quantities and syntax token names to specific values. For example, the SGML declaration in the HTML DTD specifies that the string that opens an end tag is </ and the maximum length of a name is 72 characters.

Prologue

Includes one or more document type declarations, which specify the element types, element relationships and attributes.

Instance

Contains the data and markup of the document.

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