HTML/CSS isn’t very technical since they are more or less just standards, which are written out there for developers anyways. So I’d actually throw out learning these things unless you want to be more front-end developer than technical writer. Yes its good knowledge, but I’m not sure its “relevant” knowledge, if you know what I mean.
Depending on what parties you are communicating between, I’d consider learning the domain logic for the area of development if possible. If your writing things about web development, I’d learn the stack and architecture aswell as general programming concepts so your not locked in to just explaining how the code works, since that’s kind of the programmers jobs hehe.
Finally, if you mainly cared about the $$$ I’d throw out all the programming logic and focus on databases and database design. Programming languages come and go, and can be interchanged, but something like SQL is here to stay, pays good bucks (if your good and know your stuff) and are generally required to be documented well, as part of the design processes in most businesses. But I’d consider it harder to get good at this sort of stuff, but once you are good, your pretty much set.