is Silverlight platform independent? I know it's browser independent.
Not yet
For web development, ASP.NET and Java can both be used to accomplish the same things and are completely cross-platform; Java running applets locally while ASP.NET runs remotely. As long as the web server is IIS, there is no issue here.
ASP.Net requires Window server so it's not platform independent. If we're talking servers. On the other hand java requires Jakarta which is platform independent as far as I know