One of the convenient features of in-browser inspectors like Firebug is that it is so easy to tweak CSS stylesheets. As you type in the attribute position, all possible attributes are shown in a dropdown list and can be picked from. On the right hand side of a declaration, input is restricted to only valid values (which for some attributes is really just a very small number of values).
I'm looking for a CSS mode that makes editing CSS stylesheets easier by offering sensible contextual completion suggestions, or that even restricts my inputs to only valid values. Does anything like that already exist?