The official HTML 3.2 specification

There are some minor internal inconsistencies in the HTML 3.2 specification.

The specification is relatively short and technical, and consulting the older HTML 2.0 specification (also known as RFC 1866) can be useful, since the current HTML 3.2 specifications can sometimes be understood only be assuming HTML 2.0 as a background document.

In order to understand the HTML specifications exactly, some fluency in reading SGML (the metalanguage used to describe the syntax of HTML formally) is required. SGML as a whole is rather complicated, and the SGML standard is only available in printed form. However, for the purpose of understanding the SGML descriptions of the syntax of HTML (that is, HTML DTDs), the following material usually gives you enough information:

• HTML Unleashed: SGML and the HTML DTD by Dmitry Kirsanov (detailed, yet very readable)
• Hyvin lyhyt johdatus SGML:ään by me, in Finnish
• Gentle Introduction to SGML, which is rather verbose and originally written for a specific context, but it really explains the ideas behind SGML well
• Background on Generalized Markup by Benoît Marchal; a nice introduction which helps you understand the difference between procedural markup and generalized markup (generic coding, structural markup)
• The SGML Web Page, especially the page SGML: General Introductions and Overviews
• SGML pages of W3C

Probably the most comprehensive source of information about SGML is the following book (which contains the SGML standard with annotations as well as tutorial material): Goldfarb, Charles F.: The SGML handbook. Oxford, 1990. Clarendon Press. ISBN 0-19-853737-9. UDK: 681.3.06.