Flash, Silverlight and JavaFX are the right competing technologies.
- http://en.wikipedia.org/wiki/Adobe_Integrated_Runtime
- http://en.wikipedia.org/wiki/Microsoft_Silverlight
- http://en.wikipedia.org/wiki/JavaFX
I'm not just being pedantic, it's that HTML5 is for the browser (not the desktop) and not particularly connected to the server-side (IIUC). However, HTML5 is competing with Flash, and the browser-based client-side aspect of the others.
But I don't claim to be an expert on HTML5, and checking http://en.wikipedia.org/wiki/Html5#New_APIs it has "Offline storage database", which is one of the key points for RIA.
That's my reasoning. Now your turn: why do you think Flash is the right competing technology?
AIR isn't one of these, its a wrapper for deploying applications based on Flash or HTML/Javascript as desktop apps.
Although HTML5 doesn't have a server-side that's necessary to compete with AIR, Silverlight and JavaFX, this gap has been filled many times by everyone. What is missing is excellent design tools - of which Adobe is the master (consider photoshop). And so HTML 5 opens an opportunity for design tools that take advantage of it (and probably a nicely integrated suite of server tools too). Adobe surely has a project/s underway to fill it. But it's still an opportunity. One nice thing about IT is that if you don't get in on the ground floor, you just have to wait 5-10 years to reinvent it for the next platform. In IT, everyone gets second chances (and third, fourth,...)