> there will doubtless need to be some alternative channel for events that need to preventDefault, most notably clicking on links that should route instead, and form submit; I’m not sure how that will work.

Naively I would expect that to be part of the rendered DOM("<a prevent-default='click'>"), or otherwise in a declarative datastructure available to the UI thread. Are there many events that need conditional preventDefault, such that the JS handling code would grow nontrivial?