background-attachment:fixed | Attachment differences

is to describe the relationship between the current layer and the background layer.

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Scroll: The picture is moved with the layer in front of the background when the page scrolls
Fixed: The picture and the background in front of the layer separation when the page scrolls when the page is scrolled up and down when the background picture is not moving, it's there.
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There is no difference between scroll and fixed when the height of the element does not exceed the screen.

background-attachment:fixed | Attachment differences