Incorrect style for custom CSS properties when extending a native element

[Sleeckx]

I presume the button in the following code snippet should have a red background? Not entirely sure if this should work like this: http://jsbin.com/jewatuvoyo/1/edit?html,output

There's a space between the classname and the attribute selector in the style that causes this :

.x-test-0 [is=x-test] {
  background-color:red;
}