[saagarjha]

Safari does MAP_JIT, I believe, so it keeps around RWX pages (edit: I think just one page). The best third-party apps can do (on non-jailbroken devices) is W^X–that is, map memory as RW, put code on it, then remap it as RX–because they cannot gain the dynamic-codesigning entitlement. Even this requires jumping through hoops, such as its own set of entitlements and specific setup dance, which makes it not available to App Store apps.