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
This text is
and stays there!
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.