[css-mixins-1] What happens when a parameter is not given?

[sesse]

The spec says:

If passed a dashed-function, the arguments passed to the dashed-function are mapped to the mixin’s arguments; if more arguments are passed than the length of the mixin’s argument list, the @apply application does nothing. (Passing too few arguments is fine; the missing arguments take their default values instead.)

However, what if the missing arguments do not have default values? I can see three different interpretations:

1. The entire @apply call is invalid, and is ignored (whether the parameter is ever used or not).
2. Missing default values are treated as no @env for that parameter at all. This means you can use them in fallbacks (env(--missing-argument, blue) will return blue), but env(--missing-argument) with no fallback will be IACVT (unless an @env with the same name has been declared higher up in the chain, possibly by another mixin).
3. Missing default values are treated as an @env with the guaranteed-invalid value, so you can use them in fallbacks like above but they will shadow any previous @env with the same name.