Learn how to build really great WordPress websites at Almost Inevitable Tutorials. There are plenty of free tutorials and free/paid courses for you to really learn about WordPress frontend development and UI/UX design.
If you're looking for something specific, you can search here. Stuff like CSS, PHP, Breakdance, mobile responsive, etc.
There's a debate on whether IE should be supported or not. When a Wordpress site is built, should it also be tested on IE? A surprising number of people say [...]
I don't know about you, but for me, mobile responsiveness is important. Like, really important. This means: no more meaningless full-width module stacking, and start stack modules only when needed!
Whether you are a seasoned veteran, or a beginner, there's probably something for you here that will help you upgrade your web design/development offeriings.