The cyan box 'horizon' is positioned absolutely 50% from the top of the page, is 100% wide and has a nominal height of 1px. Its overflow is set to 'visible'.
This text is
DEAD CENTRE
and stays there!
The red 'content' box is nested inside the 'horizon' box and is 250px wide, 70px high and is positioned absolutely 50% from the left - but has a negative margin that is exactly half of its width, -125px. To get it to centre vertically, it has a negative top position that is exactly half of its height, -35px.