The optimal version of a document can depend on the user's need and the
presentation device, and can govern choices of content as well as style.
The content needed for multiple versions of a document can be included in
a single document structure and identified by means of tags or by a
content selection node. Tags allow the user to specify a desired version,
while selection nodes allow the content to be automatically selected,
styled and formatted to optimize a set of constraints. Content
alternatives may be generated automatically from original content.