When user specifies a wrong image url in CSS, like:
-fx-background-image: url("/doesnt/exist.png");
there is no immediate error reported to the user. The image loading fails, error state and FileNotFound exception are stored to the image and it is then passed to FX as if everything was fine. Render engine has then problems rendering not-laded image and it's hard for user to understand what's going on, especially when they don't have the image in hands at all. CSS should check the image loading result and report an immediate error.