Suppose your task is to write a cross platform app for iPhone, Android and other modern mobile OS; for example, a mobile client for your web service.
What will you choose: to study the intricacies of the native SDK to build the UI for each platform, or use WebKit and HTML5?
Just note that HTML5 in no way restricts the access to all the functionality of the device since the WebKit engine and for iPhone and Android allows you to interact with native code and javascript. The question relates only to design the user interface — layout's, widgets, animations, various graphical elements and the like.
