Dude you'll always have to do something with HTML/CSS/JS. Even if its not your main gig. Luckily all three are fairly simple. Just go make a simple webapp without any libraries and from that you'll pick up 99% of what you'd need to know.
There's a lot of software in the world that isn't web based. Hardware, operating systems, databases, AI, libraries, languages, compilers. There isn't a computer on this planet that functions without these things.
The comment about how easy it is to learn is spot on, in my experience. I was very surprised at how fast I ramped up on vanilla JavaScript and Angular.
"That is a thoroughly web-centric view of of the world." oh geezzz..... ◔_◔