What? I think we are talking past each other. "allowHighRes" = "automatic backing store size selection by the browser according to resolution conversion from CSS pixels to backing store size" which is another way of saying "allowHighRes" = "broken programs"
"allow" is an "opt-in" kind of thing. Nothing will automatically break because you introduce an opt-in mechanism. On any account, you currently have no reliable way to query the native resolution, so even if you wanted to do the "allow" thing yourself by converting width/height transforms and setting corresponding canvas.width/height, you can't.
You will either have to create a reliable API that helps you query the correct factor by which to multiply your canvas.width/height setting, or you will have to introduce some attribute ala "allow this or that", I really don't fundamentally care, because it doesn't really matter if your conversion calculation flows to the canvas, or you derive your conversion calculation from the canvas. It's funcationally equivalent and you're just arguing about semantics.