This box is 20% wide and is always vertically centered within its parent, even if the view port changes (resize your window to see the result).
This box is 20% wide and is always vertically centered within its parent with a -50px offset, even if the view port changes (resize your window to see the result).
This box is 20% wide and is always vertically centered within the specified dark grey parent, even if the view port changes (resize your window to see the result).