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

The P generates a block box, with several inline boxes inside it. The box for
"emphasized" is an inline box generated by an inline element (EM), but the other
boxes ("Some" and "text") are inline boxes generated by a block-level element
(P). The latter are called anonymous inline boxes, because they don’t have an
associated inline-level element.
Such anonymous inline boxes inherit inheritable properties from their block
parent box. Non-inherited properties have their initial value. In the example, the
color of the anonymous initial boxes is inherited from the P, but the background
is transparent.
If it is clear from the context which type of anonymous box is meant, both
anonymous inline boxes and anonymous block boxes are simply called anonymous
boxes in this specification.
There are more types of anonymous boxes that arise when formatting tables
[p. 248] .
9.2.3 Compact boxes
A compact box behaves as follows:
If a block [p. 97] box (that does not float and is not absolutely positioned
[p. 113] ) follows the compact box, the compact box is formatted like a
one-line inline box. The resulting box width is compared to one of the side
margins of the block box. The choice of left or right margin is determined by
the ’direction’ specified for the element producing the containing block [p. 96]
for the compact box and following box. If the inline box width is less than or
equal to the margin, the inline box is given a position in the margin as
described immediately below.
Otherwise, the compact box becomes a block box.
The compact box is given a position in the margin as follows: it is outside (to
the left or right) of the first line box [p. 105] of the block, but it affects the calculation
of that line box’s height [p. 141] . The ’vertical-align’ property of the compact
box determines the vertical position of the compact box relative to that line box.
The horizontal position of the compact box is always in the margin of the block
box.
An element that cannot be formatted on one line cannot be placed in the
margin of the following block. For example, a ’compact’ element in HTML that
contains a BR element will always be formatted as a block box (assuming the
default style for BR, which inserts a newline). For placing multi-line texts in the
margin, the ’float’ property is often more appropriate.
The following example illustrates a compact box.
A compact box example
DT { display: compact }
DD { margin-left: 4em }
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