Imagine that i want to create a background image without set it into the html,body rules, but declaring it as a <div>
The problem is that this div will occupy the entire page and subsequent elements will go under it (out of page). So, it's would useful to make the background a "not-occupying" element, so that other elements can use his space.
There is a special rule in the css for do that? Hiding an element, it's possibile to set "display:none" rule, but is it possibile to do a similiar thing with the element regularly displayed?
The example is irrelevant. What you are asking for is not logical. An element cannot be in two mutually exclusive states at the same time. It either displays or it does not. If it displays then it must either be the background for a declared entity or the content of a declared entity.