Dai creatori di ExtJs nasce Sencha Touch. Sencha Touch è il primo framework HTML5 per dispositivi mobili nato dall'unione tra ExtJs, jQTouch e Raphael. Di seguito citiamo dal sito www.sencha.com la presentazione del prodotto:

Sencha Touch is the world's first app framework built specifically to leverage HTML5, CSS3, and Javascript for the highest level of power, flexibility, and optimization. We make specific use of HTML5 to deliver components like audio and video, as well as a localStorage proxy for saving data offline. We have made extensive use of CSS3 in our stylesheets to provide the most robust styling layer possible.

Altogether, the entire library is under 80kb (gzipped and minified), and it's trivial to make that number even smaller by disabling unused components or styles.