http://dev.w3.org/fxtf/geometry/#DOMRect

[[
[Constructor(unrestricted double x = 0, unrestricted double y = 0,
             unrestricted double width = 0, unrestricted double height = 0),
 Exposed=(Window,Worker)]
]]



The conscructor attributes use default values without optional.
I guess the following was meant:

[[
[Constructor(optional unrestricted double x = 0, optional unrestricted double y = 0,
             optional unrestricted double width = 0, optional unrestricted double height = 0),
 Exposed=(Window,Worker)]
]]

According to the WebIDL specification default values can only be used for optional arguments.