2.2.22 [CSS-Level2-2009] Section 9.4.2, Inline formatting contexts
The specification states:
A line box is always tall enough for all of the boxes it contains. However, it may be taller than the tallest box it contains (if, for example, boxes are aligned so that baselines line up). When the height of a box B is less than the height of the line box containing it, the vertical alignment of B within the line box is determined by the 'vertical-align' property. When several inline level boxes cannot fit horizontally within a single line box, they are distributed among two or more vertically-stacked line boxes. Thus, a paragraph is a vertical stack of line boxes. Line boxes are stacked with no vertical separation and they never overlap.
Quirks Mode and IE7 Mode (All Versions)
The following clarifications apply:
When the height of the inner box is less than the height of the line box containing it, the height of the inner box is clipped and is not preserved.
Characters are not placed in the center of the inner box when they are displayed, but are clipped from the bottom up.