In my opinion, some important technical skills for a Front-End/UI Developer are:
a) HTML & CSS: HTML & CSS skills are an obvious prerequisite for any Front-End Developer. As of this post date, HTML5 & CSS3 are the latest versions. Get to know them on an intimate level.
c) Frameworks & Libraries: There are simply too many front-end frameworks and libraries for me to list here, but some of the popular ones at the time of this post include.
UI or JS Hybrid Libraries/Frameworks:
c11) Zurb Foundation
c16) HTML5 Boilerplate
c17) Semantic UI
d) RESTful Web Services: Although it is nothing new, REST is a very popular modern web architectural style that standardizes the transfer and consumption of resources (such as data, images, etc.) between different applications, languages, or technologies. It is almost always used within the context of HTTP.
6) Design Skills or Aesthetic Sense: You don’t have to be good (or even decent) at drawing and illustration, but a sensibility for good design and usability are essential factors that round out a Front-End/UI Developer’s repertoire. You should have a basic understanding of design and expert-level ability to transform design guidelines or requirements into aesthetically pleasing websites and applications that are easy to use. Current tools of the trade (these are used more often by UI/UX Designers rather than Developers):
a1) Adobe Photoshop
a2) Adobe Illustrator
a5) Axure RP
a7) Balsamiq Mockups
Although it is not absolutely necessary for a UI Developer, I do suggest obtaining a basic understanding of the following topics:
Databases (SQL & NoSQL)
Networking Principles (specifically, HTTP)