Most people in the tech space already believe that HTML5 will replace flash when it comes to online video and UI. What's more of a toss-up is whether it will replace Flash when it comes to online gaming.
Apple, Microsoft, Google, Mozilla - four of the largest tech companies are actively pushing HTML5 in all areas, including gaming. More importantly, their products account for 97% of browser usage.
Here's why the competition & the collaboration of these companies will lead to HTML5 overtaking Flash for browser-based gaming (and who knows, maybe PC gaming in general, but let's not get ahead of ourselves).
Each Company is Constantly Improving the Technology
Apple has done wonders with HTML5 and Canvas on mobile devices. If you take a look at the results of recent performance data, you'll see Mobile Safari performs much better than other mobile browsers in canvas framerate.
They've been very vocal about their strong dislike of Flash as they have not included it in Mobile Safari, instead opting to aggressively push HTML5 as an alternative.
IE9 and IE10 surprisingly aren't terrible. When it comes to 2d
They're also behind Build New Games, a blog focused on HTML5 games
They manage HTML5 Rocks, a resource/enthusiast site for all things HTML5.
Mozilla is in the process of releasing the Mozilla Marketplace, which, similar to Windows 8, allows HTML5 apps and games to seem native (it 'installs' it to the desktop and shows up alongside all of your other programs)
Their new Mobile OS is HTML5 based (and I'm sure they'll be tying in the Mozilla Marketplace)
Last but not least, they are the team behind BrowserQuest, a very impressive MMO available for anyone to play, right from their browser.
Competition is GOOD
The flip side of things: Adobe
Adobe is a pretty awesome company, don't get me wrong. They've developed a bunch of great software: Flash, Photoshop, Illustrator, Dreamweaver, etc.
You could definitely say as of right now, the top HTML5 game development environments are at best on par with Adobe. However, you could have also said the same thing about Internet Explorer in the early 2000's. HTML5 is on an upward trend in adoption, browser support and availability of games thanks to the support of great companies that have been working hard to improve both the development process and actual gameplay.
Here are just a few of those companies:
There are countless others as well, including us (Clay.io) -- we're making it easier to implement features like leaderboards, achievements, analytics, social integration, payment processing, etc into HTML5 games (check out our developer info page)
Adobe has moved more towards open source in recent years, which is great, but Flash Player is still closed source -- Adobe is the sole company responsible for its further development.
iOS is much more strict with their apps with an approval process and chunk taken out of all revenue. We (Clay.io) also have this model, we take a 20% cut on paid games in our marketplace. The beautiful part is, however, that developers are more than welcome to try and sell it on their own with no cut taken. If they feel we're not worth 20%, they aren't forced to use us.
HTML5 is completely open - developers can choose where to put there games or just sell directly, and the advancement of the technology doesn't rest solely on one company. You see this with PC games - sure, a lot go to Steam since that's where the audience is, but games like Minecraft have done quite well.
With the recent announcement that Flash is no longer integrated on Android, Flash is dead on mobile (in web browsers that is). One thing these four companies have that Adobe doesn't is a mobile operating system, and I'm sure they would all rather see an open technology become the standard rather than one controlled by a single company (as has been proved so far).
Unfortunately, one of the most underappreciated (by game developers) abilities of HTML5 is the fact that it works with relatively few changes on mobile devices. Sure, Flash works, but you're restricted to each platforms App Store. With the mobile web you don't have to worry about app store fees on any income you generate. I would really love to see more HTML5 games take advantage of this capability instead of just developing for traditional browsers.
One issue with having so many large companies backing a technology is egos can get in the way. This has shown up in a few instances, notably:
WebGL - Everyone but Microsoft is in agreement that WebGL is the way to go for 3d graphics in a browser.
Audio - The audio tag is supported in all modern browsers, they're just not in agreement on the codec to use
Even if you're still not convinced, either way you look at it, HTML5 is a good thing. Either a) It will provide some much needed competition to Flash, or b) Take over Flash and be all-around better because of the many companies supporting it.
It's always good to hear both sides of the story, so for those who think Adobe can continue to have the success its had with Flash for gaming, make your voice heard here.