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pack
Type: one of the values below
The pack attribute specifies where child elements of the box are placed when the box is larger that the size of the children. For boxes with horizontal orientation, it is used to indicate the position of children horizontally. For boxes with vertical orientation, it is used to indicate the position of children vertically. The align attribute is used to specify the position in the opposite direction. You can also specify the value of pack using the style property -moz-box-pack.
start
Child elements are placed starting from the left or top edge of the box. If the box is larger than the total size of the children, the extra space is placed on the right or bottom side.
center
Extra space is split equally along each side of the child elements, resulting the children being placed in the center of the box.
end
Child elements are placed on the right or bottom edge of the box. If the box is larger than the total size of the children, the extra space is placed on the left or top side.

See also

More on box packing in the XUL Tutorial