Just paste the following xaml into the XamlPad shipped with Microsoft Windows SDK or the XamlPad I create myself, you will get some unexpected surprise, really cool indeed:
<Viewbox xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Margin="10"
OpacityMask="#99000000">
<Button/>
</Viewbox>
This is what I get in my own XamlPad:
