You’d need to specify it once, but not each time you use it. What’s going on is you don’t use the
prefixer mixin, you use (e.g.) the
border-radius mixin, which knows which prefixes to specify when it calls
This probably isn’t something you would want to do, when writing your own—after all, there’s no point in copying bourbon’s solution word for word. Just use it if that’s how you want it done. But, depending on how long you want your mixin to last, how flexible/ extensible you want it to be, it would be a good approach: breaking tasks into smaller units and delegating.