In general this looks good, but i am a bit concerned about allocations - introducing a wrapper for each attribute value. Does it have advantages over Boolean | String apart from userfriendliness? :)

I hear you. I would prefer, however, not to use js.|, just like I have been avoiding js.Array, js.UndefOr etc. My impression is, perhaps incorrectly, that they should be reserved for interop scenarios. They also take away from the pure Scala feel in my opinion, which is very subjective of course.

In any case, it would be great to have a set of benchmarks to be able to measure these things so that we can make an informed decision.

I understand and totally agree that we need benchmarks for a good decision 👍

For me, I am actually using quite a lot of native types (like js array), because they should be quite optimized in today browsers - and served me well when measuring performance in a huge webapp developed with outwatch.

Another strategy here, for keeping a friendly user API while avoiding allocations could be to use an encoding like Akka's OptionalVal.
https://github.com/akka/akka/blob/30749205a214e6e38fafe2636ea685b4c775fff2/akka-actor/src/main/scala/akka/util/OptionVal.scala

Internally, we would have:

class AttrValue(val v: String | Boolean) extends AnyVal

and then expose the ADT "members" as apply and unapply methods.