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In addition to the Open Source Flex SDK, there are useful tools for enhancing the application development experience such as the debug version of the Adobe Flash Player runtime and Adobe AIR for deploying applications outside of the browser. There are also professional IDE's such as Adobe Flash Builder FDT, Flash Develop and more. 1 source

The Flex applications can be written in ActionScript and/or MXML (ie Macromedia XML).

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There are tutorials online on how to configure and deploy your application to the Mac App store but it is not officially supported at this time.

Where is the Flex and AIR documentation?

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The official documentation is here

Where is the ActionScript reference documentation?

The official documentation is here

What is HTML5?

How does Flex compare with HTML5 / JS?

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Is the Flash Player open source?

No.1 http://sourceforge.net/adobe/flexsdk/wiki/About/