Why? Because we were getting people pissed off that they didn't find out until they went to sign up. We flat haven't had time to do a more complex and friendly/informative solution, but we want to. Forgive us.
Here's the backstory on the websockets decision on an app level: http://andyet.net/blog/2011/nov/14/we-shipped-an-app-that-re...
"The 10,000 people on the planet that care about Web Sockets are not your customers, and while using Web Sockets might get you mentioned on TechCrunch, supporting only Web Sockets is a great way to cut your potential audience in half."
But it all depends on who you're looking to as your market. I'd go as far to say that the group of people who DON'T support web sockets is exponentially bigger than the ones who do...
Pragmatically, though, you probably want to eventually have a fallback mode in this case, with a note telling people that they should probably use a better browser. Team apps are one of those least common denominator problems, after all.
Lack of IE9 support is an irritant (one fixable with fallback, bout your business, your call). But that annoying back button issue that resulted in me having to close my browser and reload just to get back to HN to make this comment is on my short list of web behaviors that make me never wish to visit a site or vendor again.
It does surprise me how many web sites, especially ones in many cases geared towards dev teams, either explicitly cut IE8/9 out of the picture, or render so horribly that they make me wonder whether they intentionally degrade the experience to be 'clever' or if they really just do not care about that much of their market share.
For the record, I have the most current versions of IE, Chrome, and Firefox on my PC, and run Win7, Win8, Ubuntu and OSX on other PCs just to make sure my web properties are useable for 95%+ of my visitors (I've cut IE7 support recently, but IE8 is still about 20% of my traffic).
Your browser doesn't (yet!)
Chrome, Firefox, and Safari support websockets right out of the box. https://andbang.com/you-need-websockets-yo
What are they doing that has to absolutely be done with sockets? I can't comment on the product they're offering but this is annoying to say the least as I can't see the site.
(I'm not saying I like IE, but it's the only browser that works great with higher DPI settings and scales properly for using a browser on a laptop with a 1080 res.)