73739e10_ur___The_Web_Standards_Project__Effect
[Declaration] background: yellow url(data:image/png;base64,…) no-repeat fixed [Effect] 
The background of the element is set to ‘yellow’. Also, there is a 64px by 64px red box in the background (represented by the PNG image in the data url, not reprinted in full). The red box is not visible as the background-attachment is set to ‘fixed’ which puts the red box outside of view.
Acid2: The Guided Tour - The Web Standards Project
Effect