W3C CSS2 Cascading Style Sheets, level 2 - instructional media + ...

Values have the following meanings when applied to non-floating block boxes:
left
The top margin of the generated box is increased enough that the top border
edge is below the bottom outer edge of any left-floating boxes that resulted
from elements earlier in the source document.
right
The top margin of the generated box is increased enough that the top border
edge is below the bottom outer edge of any right-floating boxes that resulted
from elements earlier in the source document.
both
The generated box is moved below all floating boxes of earlier elements in
the source document..
none
No constraint on the box’s position with respect to floats.
When the property is set on floating elements, it results in a modification of the
rules [p. 112] for positioning the float. An extra constraint (#10) is added:
The top outer edge [p. 82] of the float must be below the bottom outer edge
of all earlier left-floating boxes (in the case of ’clear: left’), or all earlier
right-floating boxes (in the case of ’clear: right’), or both (’clear: both’).
9.6 Absolute positioning
In the absolute positioning model, a box is explicitly offset with respect to its
containing block. It is removed from the normal flow entirely (it has no impact on
later siblings). An absolutely positioned box establishes a new containing block
for normal flow children and positioned descendants. However, the contents of
an absolutely positioned element do not flow around any other boxes. They may
or may not obscure the contents of another box, depending on the stack levels
[p. 125] of the overlapping boxes.
References in this specification to an absolutely positioned element (or its box)
imply that the element’s ’position’ property has the value ’absolute’ or ’fixed’.
9.6.1 Fixed positioning
Fixed positioning is a subcategory of absolute positioning. The only difference is
that for a fixed positioned box, the containing block is established by the viewport
[p. 96] . For continuous media [p. 79] , fixed boxes do not move when the document
is scrolled. In this respect, they are similar to fixed background images
[p. 188] . For paged media [p. 175] , boxes with fixed positions are repeated on
every page. This is useful for placing, for instance, a signature at the bottom of
each page.
Authors may use fixed positioning to create frame-like presentations. Consider
the following frame layout:
113