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This page tests background-attachment. How is the background image attached to its element? Does it scroll with the element, the document, or not at all?
The background image scrolls with the document. And no, the name of this value has never made any sense, but it seems we’re stuck with it.
Background is fixed relative to the entire window. The element merely serves as a viewport through which you look at the background image.
In order to see the fixed background in action, scroll the main window slowly.
The background scrolls with the element.