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.