Depends what you mean by "alternative browsers". iOS allows alternative browsers but not alternative engines (in that javascript executed on-device has to be executed through webcore). Thus Opera Mini is a completely independent implementation (all of its interpretation and rendering is done on Opera's own server), though most alternative browsers (and there are many) are UIWebView wrappers with additional features.
What iOS does not allow at all is setting an other browser as default.
> not sure if Windows RT does?
It's unclear. It definitely does not allow JITs (WinRT doesn't expose VirtualAlloc or VirtualProtect), which iOS does not allow either, but I don't know if a JIT-less completely independent browser implementation would be allowed.
If there's no way to add features to the underlying core then it's not really much better than just an alternative browser skin. A JITless browser is not going to be much good for performance sensitive applications either.
This is enough power to enable the OS vendor to gag browser apps into being second class citizens effectively.
Depends what you mean by "alternative browsers". iOS allows alternative browsers but not alternative engines (in that javascript executed on-device has to be executed through webcore). Thus Opera Mini is a completely independent implementation (all of its interpretation and rendering is done on Opera's own server), though most alternative browsers (and there are many) are UIWebView wrappers with additional features.
What iOS does not allow at all is setting an other browser as default.
> not sure if Windows RT does?
It's unclear. It definitely does not allow JITs (WinRT doesn't expose VirtualAlloc or VirtualProtect), which iOS does not allow either, but I don't know if a JIT-less completely independent browser implementation would be allowed.