Where to start? : Developing rich web applications for mobile platforms (against platform specific standalone apps)

I am new to mobile app development and I dont want restrict myself to one platform.

Some days ago I read an article that stressed on the fact that soon web apps running on mobile browsers may replace dedicated apps (at least for non-game apps). I want to develop a social networking web app that any phone can run via the browser. These are the things I would like expert comments on:

• Resources/Guidelines for developing great UIs for rich mobile web apps (HTML5?, existing APIs/Frameworks I should know)
• The best ways to increase performance (a reusable native simple browser app per platform that caches, provides a uniform 开发者_开发技巧interface to the web apps, access to sensors, APIs, features like drag drop etc for that platform. is there already one?)
• How do I test the apps for various major platforms and across various devices (small/large screens, touch phones etc)
• Best ways to manage multiple device compatibility.. (how to best handle non-uniformity of functionality on various devices/browsers on the server side using ASP.NET)

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I had similar questions to answer for my company, please have a look at my article: Testing Mobile Web – phones and platforms to focus on.

In short:

• Html 5 is now well supported on iPhone, Android and Blackberry 6.0 platforms.

• I strongly recommend to look at jQuery mobile project, especially at platform page and John Resigs excelent presentation there.

• For all of major platforms (iPhone, Android, Blackberry, Symbian) you have emulators with default web browsers

• mobile web usage statistics should always be your guide, look at StatCounter stats.

More on this you will find in my article as well as a collection of useful links for further reading.

Good luck :)