Yes, it is harder than supporting Firefox. Lean Designs makes heavy use of the canvas element and it would be impossible to implement on a browser like IE8 tha

Yes, it is harder than supporting Firefox. Lean Designs makes heavy use of the canvas element and it would be impossible to implement on a browser like IE8 that doesn't support it. There are libraries like excanvas which help, but they're very limited (no toDataURL() and the like). In order to support IE8 I'd have to severely limit what the app can do. Since this isn't an issue for 95%+ of the people visiting the site, it doesn't make sense to support it.