Can't you just build on HTML and JavaScript api's that are widely utilized and accepted? I know before I call something I'm unfamiliar with I'll look up what browsers support it. Most of the problems with IE versions have to do with plugins be it ActiveX or a Java applet. In my mind that isn't a web app but an application packed in a web browser (often so the consultants can call it a web app, "you go to a website to lauch it, see?”)