The CSS Reference Guide has many details about properties supported by each class of Node, as well as the substructure of skins of some of the controls. We need to determine the level of release to release compatibility we are supporting with this information. For example, if the substructure of a skin changes, this may mean that user-written stylesheets will no longer apply. Is this considered an incompatible change? We may also need to consider what to do if we change CSS properties to be compatible with CSS3.