There's a very specific pill shaped corner that can be made with border-radius that can only be done reliably with px in my experience.

When trying to get the same shape at different element sizes (or when you don't yet know what the size is going to be) it's common to see this hack:
border-radius: 99999px;
I'm wondering about the possibility of a value keyword for this shape something like border-radius: pill;
https://drafts.csswg.org/css-backgrounds-4/#corner-sizing