One more thing about positioning. z-indexing only works with elements that have a declare position like absolute or relative, and how that element reacts to a declare position will depend on whether or not it's parent element also has a declare position.
will not work without ALSO giving it a declare position.
But the actual display of #flash will very depending on whether or not it's parent container (<div>) also has a declared position.
That is, if it's parent container DOES NOT also have a declared position, the child <div> will do 1 of 2 things.
If position:relative, it will position as desired but space will be left as though that element was still in the natural flow. So you may move the 500 X 100 element down and over, but a 500 X 100 space will be left, as though it was still there.
If position:absolute, child is positioned relative to browser window and that position does not change even if window contracts or expands.
So be sure to give a declared position to both the parent and the child <div>.