This example demonstrates responsive design. There is two overlays: one which is sized and positioned with absolute length units, and one which uses relative length units and therefor behaves responsive.
Play with the developer tools to resize the web page or change your browsers window size to see the effect. If you set the size of the web page to 1920 x 1080 pixels (or you enter fullscreen mode with that screen resolution) you will see the two overlays converge.