Many editors provide intellisense, syntax highlighting, etc. For
example, Vi, emacs, Visual Slickedt are a few.
In addition, Dreamweaver provides an excellent WYSWIG layout tool for
HTML and DHTML components.
Programming Java Script for visual GUI elements is really a process of
leveraging / combining integrated technologies
(HTML/DHTML/Images/Css/JavaScript/Flash/etc) and attempting to do so
without boxing yourself into supporting only one browser.
With the speed that new proprietary features are implemented in the
browser population, and the proprietary way that even standard
functionality is implemented in the browser community, it might be too
much of a moving target for someone to build a specific GUI tool for.
Good luck. If you do find such a tool post back, as it would be
interesting to see how someone implemented such a beast
Regards, Scott Burkhalter