An element whose
position property has the value
relative is first laid out just like a static
element. The rendered box is then shifted vertically (according to the
bottom property) and/or
horizontally (according to the
As far as the flow is concerned, the element is still in its original position. If the relative shift is significant, it will leave a “hole” in the flow, in which case the rendered box may overlap other content.
left can be used to
specify by how much the rendered box will be shifted. A positive value
means the box will be shifted away from that position, towards the
opposite side. For instance, a
left value of
20px shifts the box 20 pixels to the right of its
original position. Applying a negative value to the opposite side will
achieve the same effect: a
right value of
-20px will accomplish the same result as a
left value of
20px. The initial
value for these properties is
auto, which makes the
computed value 0 (zero)—that is, no shift occurs.
Evidently, it’s pointless to specify both
right for the same element, because the position will
be over-constrained. If the content direction is left to right, the
left value is used, and
be ignored. In a right-to-left direction, the
value “wins.” If both
top will be used and
bottom will be ignored.
Since it’s only the rendered box that moves when we relatively position an element, this positioning scheme isn’t useful for laying out columns of content. Relative positioning is commonly used when we need to shift a box a few pixels or so, although it can also be useful, in combination with negative margins on floated elements, for some more complex designs.
One side effect of relative positioning is quite handy: a relatively
positioned element is “positioned,” which means it becomes the
containing block for any absolutely positioned descendants. This gives
us an easy-to-use tool for controlling the position of our containing
blocks: just set
relative without moving the box at all.
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