[Drup]

This is no different than trusting the processor for running the emmited assembly according to the model you have for what assembly is supposed to do.

Here, the compiler emits (a subset of) javascript which are assumed to have a semantics, and we trust the browser engine to execute it according to the semantic.

Of course we don't assume the semantic of javascript to be sane, we just assume it follows the spec (and we emit accordingly).