Do not forget to have a look at e.g. the WCAG 2.0 Guidelines. In my opinion, every UI/UX developer should have at least a basic overview about the topic web accessibility. They include basic rules about UX which helps every user, even users without any disabilities :-) If you are interested in the topic, I can recommand this checklist, which includes the main parts of the official guidelines: https://www.wuhcag.com/wcag-checklist/
Robert van der Elst
Front End Designer
Basically the tags you put along with this post ;)
Great knowledge of HTML/CSS is a definite must, since it's the foundation you work on. Javascript for the interactive side (think along the lines of jQuery, everything extra might be handy, not necessary). Knowing a bit about CSS animation gets more important I feel (still my weak spot)
Those are the programming/markup skills you need. Lots to find online obviously :)
Aside that, you need to get an eye for good design and usability.
Other than that it's practice, practice and more practice. Oh and reading a lot and looking what other people do :)
I'm still learning too, I've rolled into the UI/UX part just 9 months ago, but I've been a Front End Developer for 15 years. It's mostly the non-technical parts that are harder to 'get' from what I've learned.
If you've got more specific questions, let us know :)