According to me here are the things an experienced UI guy should know about :
Understand that CSS frameworks make our lives easier, but they also have shortcomings. You should be able to modify/customize a particular framework to suit your needs.
Today a UI developer doesn't just write CSS to implement prototypes. They must know about JavaScript and relevant frameworks like Angular, React, Meteor etc. This is because today almost every company uses a JS framework and nobody writes JavaScript the old way. Knowledge of Node is definitely a plus because you will use a lot of modules like gulp, browserify etc as a part of your workflow.
You need to be honest. If you see the prototype and don't like a particular thing it's better to let your teammates and boss know about it.
After working on UI for several years they develop good (if not great) UX skills. They start taking responsibility and come up with alternative UX strategies if they don't like something. Great UI developers know how to keep things sweet and simple.
Last but not the least - A good team player. I have seen a lot of conflicts between UI and UX guys. You need to give and take feedback and be a nice person to work with.