> You'll just spend a huge amount of time building your own implementations of all high-level functionality that is already provided by the OS and/or the browse

> You'll just spend a huge amount of time building your own implementations of all high-level functionality that is already provided by the OS and/or the browser Not only that, but you can't make a 100% guarantee that your implementation will look and work exactly the same as the native one on the underlying OS. For instance, I can re-create all the native Windows UI controls and re-implement all their behavior in exactly the same way, but what if the user has a custom theme installed? Everything breaks. (WPF has a similar problem.)