OK, but I don't recognise this from my scant experience with Tailwind, which consists of either having a framework do whatever it does with ":tailwind" (maybe it's a compiler, I don't know) which it has done without me noticing anything at all or putting a link in a link-element to pull in a CSS.

So I'm wondering whether this compiler/preprocessor stuff is actually something people run into and my experience is deceptive, or if it's something happening to few people, on the margins, that I'll likely never experience.

Because it matters to me, since I'm not going to spend time digesting the Tailwind docs and whatnot and will forever stay a casual, disinterested user that cribs some stuff I don't understand from search interfaces. If I can't expect this to continue working as it has I'll have to figure out a way to ditch the Tailwind stuff I'm already using.

Yes, they run it - but for most it is melted into another bundler like PostCSS. The few of us who try to resist the "just use bundlers for everything and everywhere" do notice that there is now a bundler process firmly anchored in your development process.

And yes, being able to jettison a pre-processor for frontend things is a very necessary thing, and unless the designers have accounted for this (I only know of create-react-app having a "jettison" feature, and then - just) you are in for a heap of fun if you need to resurrect a 5-years old app with its dev environment.