edit: Some people have asked why I didn't like XAML. My pragmatic concern has to do with the workflow and design of UI elements. You shouldn't need blend to edit validation styles efficiently. I know you don't need blend, but you can't just go in and add a single line to change the color of a border--the alternative to blend is to (afaik) copy the entire style resource that the element uses from MSDN, dig into it to find the appropriate 10 lines of useless markup that surround one word property.
CSS is infinitely more simple than XAML style resources.
I'm also very partial to the pseudo separation of concerns where HTML is for semantics, and CSS is for style, and JS is for effects. Tying all of those things into one XML based language just doesn't make sense.
That said, the whole MS C# stack has really been great to work with outside of XAML and its default elements.
