How to have content scroll over background

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I think you’re looking for css’s fixed background:
W3 background definition

Here’s an example from W3.

Something like this in CSS should work:

background: transparent url(images/bg.jpg) no-repeat fixed;

And if you want the background stretch/scale 100% width and height of user’s screen, you should check some examples in here.
See SolidSmile responde here.