I have to present a prototype of an web app in the following days to one of my clients, the thing is I\'m not so good at CSS and worst of all I\'m almost never happy with the re
There are a few frameworks aimed at (G)UI Design; Qooxdoo, JQuery UI and MochaUI being a few of them (although the last is more a proof-of-concept than a usable framework). These frameworks usually offer a variety of JS-powered elements (form elements, such as input fields and submit buttons, but other elements like tabs as well). However, it will still be up to you to position these elements, and perhaps style them, to your liking.
Perhaps familiarizing yourself with a CSS framework (such as 960GS) might complement the above JS UI Frameworks.
(As a personal disclaimer; I have very little experience with any of the frameworks mentioned above. But I'm sure either Google or SO can provide answers I can't.)