[css-mixins-1] Use the <syntax> Production for param types

[andruud]

No description provided.

[andruud]

This is an "optional add-on" to the first css-mixins-1 text which makes typed parameters a lot less noisy (--f type("<length>") vs --f <length>). Please have a look.

It should probably eventually go into either css-properties-values-api or css-values, though.

[mirisuzanne]

This looks good to me generally. Left a couple comments/questions.

[mirisuzanne]

Is the order important for this example? I don't think it should be, but not sure if we need to clarify that.

[andruud]

The order matters when the thing using the syntax definition (and the values produced using that syntax definition) wants to act differently on identifiers vs colors. For example, a registered custom property with "red | <color>" would compute red to red, but blue to rgb(0, 0, 255).

For custom functions I don't think it actually matters, but I wrote this section with the intent to eventually move it somewhere more general.

[mirisuzanne]

makes sense.