Answer: Simplicity is one reason to use HTML5 web apps, as opposed to native apps built specifically for each mobile device. Apps built for Apple, Android, and BlackBerry are not inter-compatible, so being able to use one single app for all devices is a huge time-saver for developers.
Question: What mobile devices support HTML5?
Answer: An HTML5 web app only requires a compatible browser, which most current mobile devices have.
Question: Where can I read more about HTML5's ability to store data offline?
Answer: Mobile Web applications in HTML5 can also store data offline by caching external dependencies for offline Web browsing and locally caching data until connectivity is restored.” (http://learnthemobileweb.com/2010/06/html5-and-mobile-web/)
Question: Where can I find best practices for using HTML5?
Answer: The W3C has developed recommended mobile web application best practices. The most recent version, released December 14, 2010, is available here: http://www.w3.org/TR/mwabp/
Question: What open source vendors provide HTML5 development frameworks?
Answer: Just about all mobility vendors of significance, open source or not are supporting HTML5 today. Here are some vendors I have come across recently - Sencha Touch is a mobile JavaScript framework that allows development of mobile web apps that “look and feel native on iPhone, Android and Blackberry touch devices.” The company provides a free commercial version, as well as a free version for open-source projects. http://www.sencha.com/products/touch/
Another product, DHTMLX Touch, is a free HTML5 JavaScript library for building cross-platform mobile web apps. “It’s not just a set of UI widgets, but a complete framework that allows you to create eye-catching, robust web applications for mobile and touch devices.” http://dhtmlx.com/touch/
Here’s an article about these and other mobile application framework products using HTML5: http://www.web-delicious.com/7-best-mobile-web-html5-framework-for-mobile-app-development/
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Kevin Benedict, Independent Mobile and M2M Industry Analyst, SAP Mentor Volunteer
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Full Disclosure: I am an independent mobility analyst, consultant and blogger. I work with and have worked with many of the companies mentioned in my articles.