[css-fonts-4] Define `value` of `src` with a valid value definition

[cdoublev]

The value field of src is defined with see prose. Would it be possible to use a valid value according the CSS value definition syntax?

I know that src must be parsed in a specific way, like <media-query-list> and other productions. So I suggest to set its value with something like <font-src> that would reference this specific procedure.