Honestly, that’s not good advice. UX and web development are related, but at the same time they’re completely different.
UI\UX (User Interface\User Experience)
These people do 2 things:
Make things pretty -> you need very good skills in photoshop or whatever tool allows you to make\edit graphics. A lot of designers use existing generic assets and modify them, but good designers build their assets based on the application. You also need to build schematics to present the design (pictures alone won’t do it). The big unmentioned factor here is that you need to have “taste”, basically your design needs to be pleasing to the eye, what colors go with what, how do they relate to the company and the marketing strategy (you also need to provide assets to marketing).
Design the user experience -> given an action to perform, what does the end user need to do to perform it. What to click on, how things are presented, how a process flows, how to handle errors. You have to anticipate how an end user will use the application and provide the best experience possible. Pro tip: the end user will always do something you don’t expect, how does the application manage that?
In some cases it involves managing feedback sessions where you present the design to a designated target, collect their feedback, and make improvements before releasing a feature to the public. This sounds easier than it is.
Developers take the designs created above and write code to make it work.
Both things require different skillsets and while they are related, you don’t see a person doing all 3 unless they’re a solo freelancer. It’s a lot of work, and it’s not efficient for 1 person to do all of that on a project. It is not an obvious or easy transition to go from one to the other, at least in my opinion. This is why companies have a development team and a design team. They work together, separately.
Having said that, it is a good thing for a designer to know CSS and HTML and developers usually have input on the user experience.
Only you can answer the question, What do you want to do?