Complex HTML forms

[George Sakkis]

I'd like to gather advice and links to any existing solutions (e.g.
libraries, frameworks, design patterns) on general ways of writing
complex web forms, as opposed to the typical {name:value} flat model.
A particular case of what I mean by complex is hierarchical forms. For
instance, a form that consists of a select list widget, where each
item of the list activates a distinct set of other widgets. Think for
example of a web frontend to CSV or SVN options and commands, where
each command has its own suboptions. The suboptions are hidden (or
even inexistent) until the user selects the specific command, and only
then they appear (typically by Javascript). When the form is
submitted, the selected options are passed in the server in some form
that preserves the hierarchy, i.e. not as a flat dict. Is there
anything close to such a beast around ?

George