If you have an iStuff mobile platform, iPhone, iPad, iPod, you may have come across references to HTML5 games. The late Steve Jobs disliked Flash intensely and pushed HTML5; Google has been pushing it also for their Chrome browser. You may think from this that we can expect HTML5 games to become commonplace - which is good news if you have an Apple platform, as they are limited from using Flash by Apple themselves, and Flash is the most common browser based and casual gaming platform.
Are we going to see a massive increase in HTML5 games?
Well, probably not for some time. HTML5 is proving increasingly popular with things like banner adverts, but those are a lot less complex than games. Even though newer versions of Adobe Flash CS suite will allow developers to export to HTML5, and Yo-Yo Games is making versions of their GameMaker software that will allow HTML5 game creation, HTML5 still is pretty inferior to Flash and, really, isn't really a suited to the typical game currently played in Flash.
Poor cross-platform compatibility
What this means is that HTML5 doesn't perform the same way - or at least in a similar way - no matter what operating system (Windows, OS/X etc.),browser (Internet Explorer, Firefox, Chrome etc.) or platform (mobile, laptop, netbook, desktop etc.) it's used on. Usually, some extensive platform checking is required so that different code is run to ensure that the player gets the best experience.
Flash can be protected quite easily. Admittedly, most types of protection can also be broken by someone who knows what they're doing, but HTML5 currently really only has the most basic levels of protection compared to Flash, making it easier to steal entire games.
Difficult to distribute
Flash games are usually made with one single file with everything required to play the game contained in it. This makes it much easier to distribute. HTML5 usually requires separate code, images and sound files in order to run.
There's no standard
HTML5 is still being developed. There isn't an offical standard as yet. This is at least partially responsible for the erratic cross-platform behaviour of HTML5. With no set of rules to obey, browser compatibility is a bit hit or miss.
It's not totally useless though
HTML5 has been used successfully for some things. Visit http://chrome.angrybirds.com for an HTML5 of the popular Angry Birds game. Note, though, that this is best viewed with Google's Chrome browser, and performance with other browsers may be erratic - which was one of the problems mentioned earlier.
Server-side games such as Farmville will work well with HTML5 as they just need to provide an interface for the players, rather than run actual game code, and aren't distributed.
Mobile games for iStuff
If you do use Apple mobile products, such as the iPad, iPod or iPhone, you are going to be much more limited for games than someone running Android. Your only real options are Apps from the iTunes store, and the occasional HTML5 game, or jailbreaking the device (which can cause other problems) so that it will run Flash. Most iStuff is capable of handling Flash; it's just set up so it won't by the manufacturer.
Harder for developers to earn money with HTML5
There is an extensive infrastructure in place that allows Flash developers to earn money from developing. For many, this is their job and they do have to eat. Sponsorships, in-game advertising, micro-transactions, sitelocks and extensive APIs, very few of which exist as yet for HTML5, all make the business of Flash game development easier.
HTML5 is improving, as is support for it, and there are many things that it does perform well at. Simple, casual games are not one of them currently, and probably won't be for some time.