This excellent tutorial, under development in mid 1999 and last revised in 2009, contains material on general SGML markup as well as tutorials on two specific languages under SGML: DocBook and HTML.
Both the FreeBSD Documentation Project and the Linux Documentation Project appear to
be fully committed to migration from the older, more limited LinuxDoc
language, to the more powerful DocBook language.
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">(The lower case string "html" for the root tag under HTML 4.01 is not actually case sensitive but this and all other tag names under the corresponding XML version of HTML must be lower case.)
If you are logged to the campus Unix network, you should use the program /usr/local/xml/bin/validhtml rather than frequently banging on the W3C validator (which might then decide to boycott you).