See intro blogpost here.
Silverlight in general seems less restrictive when it comes to your XAML. For instance case sensitivity. I was recently trying to use a class modifier on my UserControl using the following:
However this doesn’t work in WPF. It turns out that the "internal" keyword must be lower-case in WPF. Luckily this also works in Silverlight, so stick with the right casing and you’re good :-)
Next: WPF vs. Silverlight - Part 8 - Reusing code in Visual Studio