HTML and CSS (just the two, yes, classic front-end stuff) are often overlook in favor of JS. Many developers reach a point where they understand the base concept for layouts, standard styling properties and the way to build elements. Often accompanied with a library like Bootstrap or Foundation.
But it turns out that there is a lot more to building layouts than just that. In fact, it’s rather easy to spot the TON of badly written front-end code. Its truly rare to see a project where things make sense, there is no useless properties, overly-complex stylings and hacks left and right to solve problems caused by bad code in the first place.
More than once I’ve deleted 300+ lines of code to write 50-60 lines that do the same job with less bugs and it’s easier to understand.
So, if you think that there is still some room to grow, try out the following projects in your spare time! Note, the order is not in terms of difficulty or anything.