:has() is at 94.41% of tracked global browser support. That's pretty good. Cascade Layers are at 96.3%.
oklch at 93% (with issues)
color-mix at 91.18%
CSS Nesting at 90% (if you're careful)
@ scope at 84.83% (0% Firefox)
light-dark() at 82.7%
custom property style queries at 73.49% (0% Firefox)
There's no magic number, and a lot we can do with progressive enhancement – but these are some of the recent #CSS features I reach for most often, and wish they would Just Work Already, Please.