Description:

Emacs is an advanced and extensible editor. An Emacs major mode customizes Emacs
for editing particular types of text documents. PSGML is a major mode for SGML
(a markup language) documents. PSGML provides several functionalities for
editing SGML documents: indentation according to element nesting depth and
identification of structural errors (but it is not a validating SGML parser);
menus and commands for inserting tags with only the contextually valid tags;
attribute values can be edited in a separate window with information about types
and defaults; structure based editing includes movement and killing; and also
several commands for folding editing.