float: left;
position: relative;
position: absolute;
top: 0; left: 0;
In Opera 7, the small red box is positioned relative to the root element
(at the top left of the viewport).
However, since the floated box additionally has position: relative,
it (and not the root element) should become the containing block for the red box,
so it should be positioned at the top left of the gray box.
→ Definition of "containing block"
This Bug is fixed in 7.5 preview 1.