try this instead:
taken from here and explained why :)
Rounded corners in WebPart headers SP2010
SPD uses 2.1 css by default and your using css3 ;) hence why your getting that error.
So you need to do the following to stop the error:
In SPD, look at the menu and select the "Tools" and then go down and select "Page Editor Options"
A new box should appear for you that you can set, what you need to do now is select "Authoring" tab at the top left.
You can now change the css validation to the correct version for the schema, yours being css3.
You will note that you dont have css3 in the list as its only 1.0 to 2.1 :) thats because css3 is not supported yet! you can have this is vs2010 http://visualstudiogallery.msdn.microsoft.com/7211bcac-091b-4a32-be2d-e797be0db210 but that is as far as it goes atm! Even tho it throws an error it should still work :)
hope it helps!