CSS is a real language, and you need deep technical knowledge to understand it. But plenty of software developers hate it and look down on it. It’s a good, if incomplete, tool for what it does. But I think it scares some of the gatekeepers who were drawn to software before the web.
It can’t be unit tested. It’s a language that only exists in a domain that stretches multiple sizes, multiple devices and multiple renderers. There’s more than 1 way to do things. And some of the biggest challenges with CSS are human. It’s the paintbrush for the bike shed.
Funny how many hard problems in computer science, including cache invalidation, are about users.