Even though the XAML defined for use in Silverlight is a unique language that is not tied to WPF XAML, the Silverlight team has made an effort to use WPF XAML as the starting place for its Silverlight XAML design. In this first release of Silverlight, all the XAML tags used in Silverlight are also compatible with WPF.
Compatibility will be a priority in future releases. However, since WPF and Silverlight are evolving in parallel, you can expect that some innovation in the markup will be first introduced on one of the platforms that is not implemented on the other.
For the WPF developer, the most glaring exclusion is controls. The design goals of Silverlight are meant to provide the maximum of functionality for the smallest download cost. Because of this design goal, no WPF controls are supported initially. This means no text boxes, no buttons, no combo boxes, and, in fact, nothing from WPF that derives from the WPF Control base class at all.
To developers (and designers), the exclusion of controls from the XAML may seem a major omission. But two key goals of Silverlight are to have a small runtime and to be cross-platform compatible, and here in its first release, Silverlight is attempting to fulfill some particular web-specific use cases, including:
Choosing these use cases means that in addition to lack of control support, ...