Can you elaborate?

I am considering HTML for a mobile game, but I am concerned that performance will be lacking and CSS will be challenging.

I built the mobile HTML5 version of Words With Friends that launched as part of Facebook's failed mobile HTML app platform, aka "Project Spartan". (standard disclaimers: I don't work for Zynga any more, these are my own personal opinions, etc).

I much prefer web development to native iOS development for many many reasons, and am 100% for building desktop games in HTML5, but the thing about mobile web apps is that you'll spend every waking minute focusing on performance; you'd better be comfortable with not using jQuery, and you'd better be ready to get intimately familiar with the intricacies of how WebKit renders the DOM so you can fine-tune by hand the way it applies hardware acceleration. If you're lucky, with tons of optimization you might get performance to a barely-acceptable level, and that's assuming your game's running in MobileSafari instead of a Nitro-less UIWebView. Things are only going to get better from here, but they've got a long way to go before I'll voluntarily write another web-based mobile game.

Also, if you're building a pure web app (as opposed to a native wrapper around a UIWebView), you functionally won't be able to have sound. I've heard that iOS6 is finally introducing a real web audio API, but the current version of iOS doesn't have a sane way to play audio in a way that's robust and full-featured enough for most games.

(And if you care about cross-platform support, don't get me started on the clusterfuck that is the Android browser. I'd honestly rather have to support IE 6/7/8 than the Android build of WebKit.)

totally agree. Android "Browser" is like the IE6 of mobile web. I bump into so many problems trying to tame android "Browser". Lack of hardware acceleration for simple displacement animation, the best transition animation you can apply is probably fade in and out not more than 2 stacked layers. The canvas was missing toDataURL method, canvas pixel manipulation with alpha channel resulted in some fuzzy pixels scramble. Weird virtual keyboard behavior. It takes few seconds for the device to tell the browser that orientation has changed, sometime has to be accompanied with light shaking of the device. No svg support for some older models. And majority of android devices out there are not running the latest Android OS. Luckily Chrome is much better browser than Android "Browser". Kudos to chrome team to make the mobile web on Android awesome again.

I would highly recommend going native for building games. With HTML5 you won't get the performance you do from a native app primarily because OpenGL is hardware accelerated and performs very well on iDevices. Plus JavaScript executes fairly slowly in UIWebViews. If you spend a lot of time you can improve the performance, but it will still be much worse than native if you're shooting for a frame rate >30fps and plan to have many elements on the screen.

Shameless self plug: If you're looking to build an iPhone game check out our website (https://www.makegameswith.us), you'll learn how to build Angry Birds in no time then get started making your own game!

Unless it's a very simple game (very little animation, no physics etc) then a game that uses the DOM (and CSS) for game objects will perform horribly.

HTML5 canvas is definitely a possibility though. Check out http://impactjs.com/

Its a great game engine which will allow you to create all manner of high-performance games on iphone4+ hardware (i've never tested on <iphone4, so performance may be ok there too)