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.<p>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.<p>Of course we don&#x27;t assume the semantic of javascript to be sane, we just assume it follows the spec (and we emit accordingly).