Well, Bootstrap 5 is ~200K when not minified and a web site usually ships additional CSS resources as well.
One such process is what you're commenting on here in this thread.
If memory serves me right, you could compile the Bootstrap library and choose the parts you needed. Not sure if that is still true for Bootstrap 5. If it is not possible, it stands to reason, that perhaps one should not use Bootstrap 5 and instead write good styling oneself, without all the stuff, that will never be used on the site anyway.
CSS can be tricky, but in the end, reasonable styling is usually not that hard to do. Even responsiveness can be done mostly without media queries these day, unless you want content to change, if layout changes. It used to be much harder, when our browser standards were not that far. Nowadays we have flex and grid and more standardized interpretations of CSS rules in browsers. We can make navigations, menus, tiles and whatever all responsive without that much work put in.
Anyway, as others comments said, these kind of tools nowadays are more 'transformers' than mere 'minifiers'.
I wonder how much ineffective styling still remains after "tree shaking". I guess the fancy tree shaking is out of luck, if we add unnecessary styling to elements, which are indeed used. It seems a really hard problem to solve automatically, to transform CSS in a way, that takes care of redundancies and unnecessary styling, while still maintaining the readability, which well crafted CSS can have.
The problem is, with CSS you don't know if you've added too much. As the project evolves, and code and styles change there's literally no way of knowing if a certain CSS rule is still in use Well, except regexp'ing the whole project looking for matches.
That's basically what these tree-shaking tools do.
Yes there is:
Tailwind actually has its own jit compiler for development since the old, full css was extremely long. Windi always creates small css as far as I remember.
When there is a need for a tool that minifies CSS, then people seriously need to ask themselves, how it can be, that they have that much CSS. How much redundant CSS can you accumulate? Was there no coherent styling idea or strategy, so that every thing on the page needs separate styling?
What many forget is, that data is often sent gzip compressed anyway, which naturally takes advantage of repeated parts to compress. Text usually compresses pretty well. Especially something like a description language with many repeating parts. It is great, that Parcel CSS is faster than some other tool. However, for me, these kind of tools are merely treating the symptoms of not doing styling properly. I'm glad, that I got to know the web, when you simply could look at all the source of a page and learn from it, instead of facing a multi megabyte "minified" script and "minified" CSS.