Sounds like a similar approach to Wicket, which remains the best web UI framework (possibly the best framework period) I've ever used.
Wicket suffered because it straddled the transition from pure HTML to HTML+AJAX. It could do HTML+AJAX, and their solution was elegant, but it was an afterthought and Wicket wasn't really designed around it.
Kweb was designed from the ground-up for this world and this affords it many advantages.
The reason is it depends on some stuff like coroutines - and since Scala deprecated their delimited continuations plugin way back when, I don't believe Scala has any equivalent feature.
May I ask why you ask?