When you build a web application using Silverlight 5 and XNA 3D, you may find that your application does not load and appears as a white block. The solution is simple.
In the area of your page where you expect to see your silverlight application, right click to bring up the Silverlight menu. Click the only menu option there is that says Silverlight. This will bring up the Microsoft Silverlight Configuration dialog.
On the Permission tab click on allow to enable 3D graphics.