HTML -- Hypertext Markup Language

General

Browsers

SoftQuad: HotMetal (see also ~/rati/SGML/HotMetal)
Hyper-G: Harmony, Amadeus, etc.

HTML - level 1

HyperText Markup Language is a SGML DTD. In practical terms, HTML is a collection of styles (indicated by markup tags) that define the various components of a World Wide Web document [ref].

This is the level mandatory for all WWW clients. Level-1 is basically the HTML of the initial WWW clients, plus images.

See HyperText Markup Language (HTML): Working and Background Materials: HTML level 1

HTML level 2 (also HTML 2.0)

This is a superset of level 1, also including forms for user input. The specification is still being refined, mainly a question of defining its expression in terms of SGML, and accurately describing current accepted practice.

See HyperText Markup Language (HTML): Working and Background Materials: HTML level 2

HTML level 3 (also known as HTML 3.0 OR HTML+): Developers only

Above level 2, HTML includes features such as tables, figures, and mathematical equations. Design Objectives [ref]:

Backwards compatibility with 2.0
Tightening up HTML.Recommended and moving more things to HTML.Deprecated
Keep HTML - simple don't compete with CALS
Make it practical for people to edit HTML 3.0 documents directly, i.e. avoid long names
Tables, figures and math from HTML+ with tweaks based on recent experience
Add limited presentational controls with a view to use of linked style sheets
Compatibility with ICADD as per Yuri's suggestions (using or perhaps FIXED attributes).

See HyperText Markup Language (HTML): Working and Background Materials: HTML level 3

Material on HTML+ (= HTML level 3, but might be old!)

HTML+ is a superset of HTML and designed to allow a gradual roll over from the earlier format, with features like tables, captioned figures and fill-out forms for querying remote databases or mailing questionnaires. Large documents can be split into a number of smaller nodes for reduced latency, with explicit or implicit navigation links. This draft also includes a proposal to add support for mathematical formulae. Authors can include limited presentation hints, and further control may eventually be possible via associated style sheets [ref].

One most notable difference between HTML and HTML+ is the use of containers. For example

is a container in HTML+ rather than a separator. This change has been made to facilitate verification, and to provide greater flexibility in specifying link destinations. The major additions over HTML are:

nested lists
inline images and drawings
embedded data in foreign formats for mathematical equations etc.
tables with support for titles, and column headings and an ability to let entries/headings span columns
forms - for querying or updating information sources and filling in questionaires for mailing or faxing

Links can now be anchored on a wide range of containers by using value of the container's id attribute as part of a hypertext link.

Things dropped from HTML

MENU and DIR which are now handled as attributes to UL
all inline emphasis is now handled with the EMPH tag
PLAINTEXT, LIST and XMP have been obsoleted
IMG (X Mosaic) now handled with FIG element

[ref]

HTML+
Keywords: technology, html+
Title: A Review of the HTML+ Document Format
Author: Dave Raggett <dsr@hplb.hpl.hp.com>
Institute: Hewlett Packard Laboratories, Bristol, UK
PostScript, Size: 847238, Printed: 10 pages
Proceedings of the first international WWW conference, May 25-27, 1994, Geneva, Switzerland
HTML+, Dave Raggett (ftp)
HTML+ (Hypertext Markup format), Dave Raggett (Nov 1993)
HTML+ Document Type Definition, Dave Raggett (April 1994)