This was already an option with Sass's @extend (later ported to a PostCSS plugin)

Cool. So @apply is basically a way to compose classes from other classes?

Yep

And then you are just writing css with a more obtuse syntax.

Congratulations, you have reinvented the purpose of CSS classes. This is why I don't use Tailwind, at a big enough scale, it becomes lots of duplication, and if you use @apply, it's just...CSS classes as originally designed.

I still think it would be nice if CSS natively allowed you to combine classes the way these @extend and @apply operators do

In practice, I never have found much benefit to that. If I need a base class, I will create a base class. .validation-message for instance, then if the validation is a success/error/warning/info, I will write those classes, then my validation message element will have 2 classes on it "validation-message warning"

That way there is no repeated rules like in the final compiled @extend/@apply operations.

Part of the problem with the tailwind debate is that the appeal of tailwind is not just having your styles in your html, it's also having predefined style units, e.g. size intervals for things like margins.

CSS variable libraries like open-props also do this, only without the controversial use of utility classes. Add editor snippets to that and you have nearly the same ease of use as tailwind.

You can't reinvent something that was never invented to begin with. "CSS classes"? ("as originally designed"?)

Class definitions are not CSS. They're markup. The CSS selector language does let you target them (with a leading dot, as in .foo)—just like CSS lets you target other things defined in markup with special syntax: element IDs (#bar), attributes ([foo=bar]), etc. It's not as if HTML existed without classes and without CSS, and then CSS was invented and with it came classes, too.

(This isn't to say that you're alone. A frighteningly high number of people reveal through their choice of language an apparent belief that `class` was invented by the CSS working group (or something). That when you're writing HTML, evidently 95% of it is HTML proper, but then when you get to the part where you type out 'c', 'l', 'a', 's', 's', that you're shifting into CSS and it's like, "This last piece with these "classes" was contributed by the CSS folks, and so that's why/how CSS leaked in," without ever seeming to realize that... they're not actually writing CSS. It's still just markup—using the ordinary syntax for specifying attributes and their values. The CSS only happens when you, uh, start writing CSS...)

[delayed]