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147). Note that even
for the top and bottom margins the percentage value will refer to the width of the
containing block. If the containing block??™s width depends on the element to which
percentage margins are applied, the resulting layout is undefined in CSS2.1.
Negative values are allowed for margins (although implementation-specific limits
may apply), and have the effect of pulling the element in the direction of the margin
Box Properties
201 Box Properties
specified. This may cause the element to overlap other elements, which may, of
course, be the desired effect. In cases where overlap occurs, we can determine the
elements??™ stacking levels by applying z-index values to them. In the case of
non-positioned or floated elements, a z-index only takes effect when a position is
set to relative for the elements, as a z-index can be applied only to positioned
elements.
Negative margins on floats are handled differently and the details are covered in
Floating and Clearing (p. 180).


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