Re: Thinking about mixins as a new type of selector

|  Is there any reason mixins/extends shouldn't use property
|  syntax? Since variables will be using property:value
|  notation, why not use it for this use-case as well.

Yes, there is. Because CSS “variables” are custom properties, meaning they 
are properties like any other: they cascade, they're inherited, they're tied 
to an element (not a rule or a stylesheet). As such, it makes sense to use 
the propery syntax for them.

To the contrary, @extend and mixins are not property declarations.