HTML 4.x

HTML 4 introduces frames as a legal entity and is the last version of straight HTML that will be valid. XHTML 1.0 is a rewriting of the HTML 4.0 specification in XML. If you understand HTML 4.0 it's a short step to learning XHTML 1.0.

HTML 4.0 Specification
W3C writes the working drafts of all HTML specifications and eventually will determine a standard for HTML 4.0.

Changes Between HTML 3.2 and HTML 4.0
Go here if you are updating older pages to use the newer tags. This site shows deprecated tags as well as the newer tags.

Deprecated Tags in HTML 4.0
These are tags to avoid, as they will become obsolete in future versions of HTML.
HTML 4.0 Conformance Test
Find out if the browser you are using supports HTML 4.0 (the newest features).

HTML 4.0 Tags
This list of tags has information about all the tags used in HTML 4.0. It includes attributes, end tags, and deprecation status.

New Tags in HTML 4.0
If you are already conversant with HTML 3.2, then this list of the newest tags will be useful.

Obsolete Tags in HTML 4.0
If you are still using these tags in HTML 4.0, they may not work.

Structure of an HTML 4.0 Document
This document gives a good overview of how an HTML 4.0 document works, including the structure of tags, how to escape special characters, and includes an HTML 4.0 validator at the end.

What's New in HTML 4
The Web Design Group does a good job of describing the latest features of HTML