when u call CefWindowInfo::SetAsWindowless set the second parameter to true, to enable transparency.
Now cef passes the alpha values u set in ur background colour or image on to the RenderHandler::OnPaint
For documentation checkout
https://code.google.com/p/chromiumembed ... ?r=506#106
the colour values come as BGRA.
I know that works, because I'm using it myself
In DX use a texture with an alpha channel and enable transparency there too.
Have fun
Markus