a:b is almost universal as a range designator at this point. / is a _terrible_ choice, especially in a context where a fractional value would also make sense.

That was just an objectively bad syntax choice, but probably keeping on track with CSS.

Don't you think : is already used for something else in CSS?

making excuse is easy, here is another one 1..3

And what’s wrong with whitespace? Like every other property.

range(1, 3) might be more consistent, it's a single value that is a range and not 2 values.

On a side note: a problem with CSS' claim to be a programming language is that the grammar is not obvious.

A single value is a choice, two values as well.