Emacs's PSGML mode: home page
It is a nice mode to edit pages.
Some of my problems
Editing without a DTD.
No schema support.
Syntax highlighting is not optimal, e.g. attributes cannot have a different color, and it is sometimes difficult to see well.
Notes
2003-02-15: the CVS version of sgml.el seems to solve the coloring and DTD problems according to a Usenet post.