Rapid UI prototyping
There are two very nice frameworks for quickly creating UI. Both of them have lots of features, excellent documentation and active community.
Twitter Bootstrap has more components and the default theme looks more suitable for web application than a site.
The default theme of Zurb Foundation on the other hand looks more suitable for a web site than for an application.
Choose whatever You like more, both of them are good and roughly the same.
You still need to know HTML & CSS but those frameworks will greatly simplify the work, also, its own sources are very well documented and are good way to learn new things and improve HTML & CSS skills.
By Alex Craft