SGML is a very important system for document storage and interchange,
but it has no formatting features; its companion ISO standard
(see http://www.jclark.com/dsssl/) is designed for writing
transformations and formatting,
but this has not yet been widely implemented. Some SGML authoring
systems (e.g., SoftQuad
Author/Editor) have formatting
there are high-end specialist SGML typesetting systems (e.g., Miles33's
Genera). However, the majority of SGML users probably transform
the source to an existing typesetting system when they want to print.
TeX is a good candidate for this. There are three approaches to writing a
lex; this is hard, in practice, because of the complexity of SGML.
Balise. They are expensive, but powerful, incorporating SGML query and transformation abilities as well as simple translation.
nsgmls, and this produces a much simpler output format, called ESIS, which can be parsed quite straightforwardly (one also has the benefit of an SGML parse against the DTD). Two good public domain packages use this method:
Both of these allow the user to write “handlers” for every SGML element, with plenty of access to attributes, entities, and information about the context within the document tree.
If these packages don't meet your needs for an average SGML typesetting job, you need the big commercial stuff.
Since HTML is simply an example of SGML, we do not need a specific
system for HTML. However, Nathan Torkington developed
html2latex from the HTML parser in NCSA's
The program takes an HTML file and generates a LaTeX file from it.
The conversion code is subject to NCSA restrictions, but the whole
source is available on CTAN.
Michel Goossens and Janne Saarela published a very useful summary of SGML, and of public domain tools for writing and manipulating it, in TUGboat 16(2).