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.