HTML 4 extends HTML with mechanisms for style sheets, scripting, frames, embedding objects, improved support for right to left and mixed direction text, richer tables, and enhancements to forms, offering improved accessibility for people with disabilities.
2.3 HTML 4
http://www.w3.org/TR/1999/REC-html401-19991224/intro/intro.html#h-2.3

It (HTML 4) provides further rules to condition and refine the use of the W3C Recommendation in a way which emphasizes the use of stable and mature features, and represents accepted SGML practice.
Introduction
http://www.cs.tcd.ie/15445/15445.html

HTML is a subset of the standard generalized markup language (SGML) and, as a consequence, is a language that emphasizes the structure of a document rather than its description or layout. It is also a dynamic language that is continuing to evolve, pushed by developers (especially Netscape and Microsoft) who integrate markup extensions into their browsers.
Hypertext:
http://lrs.stcloudstate.edu/web/tutorial/

HTML is the name of the set of codes used in writing the vast majority of documents found on the World-Wide Web.
HTML Tutorial -- Writing Web Pages - Introduction
http://www.crosswinds.net/~msh210/html.html

HyperText Markup Language is a way of adding various attributes to simple text files that are displayed on the World Wide Web.
HTme Create a Site - What is HTML?
http://www.htmltutorials.net/html_definition.html

UAWeb Resources -- Standard HTML Tags
http://uaweb.arizona.edu/resources/tutorial/html/standard.shtml

Simple and easy to understand. Well organized. There are good examples.

HTML - The HyperText Markup Language
http://www.wdvl.com/Authoring/HTML/

A lot of resources, and well detailed. Each section is written by defferent author so that it is not well organized.

HTML Tutorial -- Writing Web Pages
http://www.crosswinds.net/~msh210/html.html

Neccessary inforamtion and well organized>

Interactive HTML Tutorial
http://lrs.stcloudstate.edu/web/tutorial/index.html

Well organized, plain web site. There are good examples.

Sunrise HTML Tutorials - Index
http://www.sunriseresearch.com.au/TUTORIAL/

Well organized and plain web site.

Outer Court - HTML4
http://www.outer-court.com/tech/htmlpt1.htm

Easy to understand. There are good examples. Good for beginner.

References

SUPER HTML 4.0 REFERENCE (Japanese)
http://www.zspc.com/html40/

Very well organized for all information for HTML4, including well detailed alphabetical or categorized tag reference. It is, however, written in Japanese.

HTML 4 Complete Reference
http://www.december.com/html/4/

Well organized for the HTML tag. This site is prrety simple, but here are detailed and enough information.

Advanced HTML Reference
http://webref.info/index.htm

Well organized and easy to see each tags. Since the table is organized for each browser(ver HTML, IE, Script, NN, Opera, and WebTV), easy to determine which one correponds to a sepecific tags. This page, however, does not include example to use.