I think extjs is to sencha what windows is to microsoft. Yes, modern browsers on desktop or pads/phones should support extjs framework. However, sencha touch is optimized for the pads/phones.
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Sencha publishes the road map for all of our frameworks under each respective products' "Road Map" tab. To learn more about our current Ext JS Road Map, visit Sencha's Ext JS Road Map page.
Distribution of a Sencha Touch application through a general purpose browser, regardless of the device, is free of charge an embedded license is not required. A general purpose web browser has a URL bar and a general browser "Chrome".
Audio Video Support:
Provides HTML5 to deliver components like audio and video, as well as a local Storage proxy for saving data offline. Provides CSS3 style sheets for robust styling layer possible.
Provides a simple Container for HTML5 Video
Easily scales to different resolutions for maximum compatibility with different iPhone, iPad, and the various Android phones.
HTML5 and CSS3 allow higher flexibility.
Great support for animations and enhanced touch events
Saving data offline with local Storage writers