To be precise, it's not that JIT is now allowed in iOS; that would be a security issue because it would basically allow sandboxed apps to create executable memory area, which is considered very bad. On the contrary, what just happens is that WKWebView is piggy-backing on the new extension system to run in a different process, with a different relaxed sandbox,
and communicates to the host app with an IPC channel (similar to eg. Chrome extensions). It looks like there are not enough features exposed over this IPC channel to make a fully featured browser, yet.
