From the spec:
10.6.3 Block-level, non-replaced elements in normal flow, and floating, non-replaced elements
[...]
If [a block box] has block-level children, the height is the distance between the top border-edge of the topmost block-level child box and the bottom border-edge of the bottommost block-level child box. However, if the element has a non-zero top padding and/or top border, then the content starts at the top margin edge of the topmost child. Similarly, if the element has a non-zero bottom padding and/or bottom border, then the content ends at the bottom margin edge of the bottommost child.
There now follows four boxes, each given a different value for the
overflow property -- the value is shown inside the boxes.
The result should be the same for all four cases, that is, a navy box
containing a red box containing a green bar with yellow text. The only
difference would be the addition of scroll bars on the last
case.
For 'auto' and 'scroll' values, Mozilla seems to eat up the top and bottom margins. (For scroll, there is also a problem with the width, we appear to be making the 'width' property include the width of the scroll bars, which may not be correct.)