A float is followed by a clear with a background (both have zero margins.)
The clear has padding-top:10px (doubled)
The clear has padding-top:20px (doubled)
The clear has padding-top:0 (OK, nothing to double)
The clear has hasLayout and padding-top:10px (OK, NO doubling)
The clear has padding-top:10px and NO background-color (OK, NO doubling)
The clear has padding-top:10px and there is something beside the float (OK, NO doubling)
The clear has padding-top:10px, there is no float (OK, NO doubling)
Related bugs: