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One funny thing about HTML5 knocking off Flash for mobile is that while Adobe was involved in its dispute with Apple over Flash, it was also developing programming tools that supported both Flash and HTML5. You have to think Adobe saw the writing on the wall if they were hedging their bets against their own baby. Of course, when you had Steve Jobs pushing HTML5 specifications, you had to assume there was a reasonable chance that it would succeed.
A difference between HTML5 and Flash/Silverlight is that HTML5 is a standard developed through the World Wide Web Consortium and has royalty free licensing. All things being equal, people are going to prefer royalty free to paying royalties. Well, except for the people that get the royalties, I suppose. Another very nice thing about HTML5, assuming it becomes essentially the standard, is that there will be one common platform instead of the complexity of developing for multiple competing platforms.
To get around this platform issue with active content, adobe had their Flash platform which they ported to each major platform. Not content with letting adobe own this ground, MS developed Silverlifght for essentially the same purposes (but failed to port this to Linux or Mobile platforms).
Fact is, developers have been chomping at the bit for a truly platform and browser independent standard that they can develop for. MS and Adobe can be said to have been deliberately stalling the creation of the required standards, and practically speaking, that was perhaps their only tactic in the face of developer and content-consumer pressure.
In my view, the final straw has been the success of mobile devices. Neither MS nor Adobe had a good mobile device solution (with Apple refusing flash, and MS being none-existent in mobile). Nonetheless, video and active pages are a must in the mobile world, and at least one of the main mobile players (Google) is more than interested in keeping standards open and fair. As mobile pages develop and get richer, we will see the standard advance, and I expect html6 to be a lot closer than html5 was to html4.