Understand The Web
Above all, this article makes you realize that making decent cross-platform applications are all but impossible no matter what you do. My favorite quotes:
If you reach the point of building a browser-based application that you depend on so many proprietary enhancements that your users can only access it using Google Chrome, I think you’ve picked the wrong platform.
I honestly think that ‘Desktop-class Web Applications’ are a fools folly. Java, Flash, AIR and QT demonstrate right now that cross-platform applications are always inferior to the functionality and operation of the native framework on a host platform. Steve Jobs is right in his comments that third-party frameworks are an obstacle to native functionality.