In https://drafts.csswg.org/css-values-4/#component-types the different component value types are defined.

And Function definitions are mostly specified separetely from where they are used, so they are referenced by non-terminal values like <image()> in https://drafts.csswg.org/css-images-4/#image-values.

So far, there is no note in the specification how the names of those values should look like. This causes some confusion like in w3c/fxtf-drafts#410.

In order to clarify that and make it consistent across specifications, I suggest to formalize that the names of those values need to be followed by an opening and a closing parenthesis.

Sebastian