well for a start try this out:
WebGL is a way to call OpenGL calls from a web page.
Silverlight is a high level language that indirectly calls the host system. on Windows it calls DirectX
*BUT* the Silverlight app does not make *DIRECT* calls to DirectX
so while Silverlight can create graphics it is not the same.
Honestly IMHO if you need to do a lot of animated graphics should it be a web page ?
if for example it's a game I would rather have a native app that runs w/o a browser in the middle.
that's just one example.