iOS binaries that are not signed by apple are not permitted to mark memory pages as executable if they have previously been writable.

This restriction makes exploiting buffer overruns very difficult on iOS - particularly important as objective-c doesn't give you much help avoiding them.

However, you can't write a runtime compiler unless you can generate bytecode (write) and then execute it, and nobody has found a way to write a performant javascript or CSS engine without some form of runtime compilation.

So, Apple does allow you to write your own browser backends, but they won't give their signature, which would permit you to use riskier techniques to gain performance.

In practice, that means any browser not using the safari engines would be unacceptably slow on the modern web.