>Has anyone checked our how browser independent ASP.NET generated forms are? What makes me ask is a 3rd party tree menu component for .NET I was looking at. Whwn I hit their demo pages with anything but IE (NS6, Opera, Mozilla), they failed miserably!
>
>Is it just a bad product, or is ASP.NET pretty dependent upon IE?

ASP(.NET) has nothing with the browser by the client. If the web-page uses client scripting, or some other client executed portion (Shockwave flash, Java applet, etc.) of the web-page. So if it is not a plain HTML it will probably be not well-supported by old or technology backward browsers.

Also... for what to worry if more than 80% of the internet users use MSIE?

You may find the following link useful:
http://www.upsdell.com/BrowserNews/res_design.htm

Regards