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

Empty inline elements generate empty inline boxes, but these boxes still have
margins, padding, borders and a line height, and thus influence these calculations
just like elements with content.
Note that if all the boxes in the line box are aligned along their bottoms, the line
box will be exactly the height of the tallest box. If, however, the boxes are aligned
along a common baseline, the line box top and bottom may not touch the top and
bottom of the tallest box.
10.8.1 Leading and half-leading
Since the height of an inline box may be different from the font size of text in the
box (e.g., ’line-height’ > 1em), there may be space above and below rendered
glyphs. The difference between the font size and the computed value of
’line-height’ is called the leading. Half the leading is called the half-leading.
User agents center glyphs vertically in an inline box, adding half-leading on the
top and bottom. For example, if a piece of text is ’12pt’ high and the ’line-height’
value is ’14pt’, 2pts of extra space should be added: 1pt above and 1pt below the
letters. (This applies to empty boxes as well, as if the empty box contained an
infinitely narrow letter.)
When the ’line-height’ value is less than the font size, the final inline box height
will be less than the font size and the rendered glyphs will "bleed" outside the
box. If such a box touches the edge of a line box, the rendered glyphs will also
"bleed" into the adjacent line box.
Although margins, borders, and padding of non-replaced elements do not enter
into inline box height calculation (and thus the line box calculation), they are still
rendered around inline boxes. This means that if the height of a line box is
shorter than the outer edges [p. 82] of the boxes it contains, backgrounds and
colors of padding and borders may "bleed" into adjacent line boxes. However, in
this case, some user agents may use the line box to "clip" the border and
padding areas (i.e., not render them).
’line-height’
Value: normal | | | | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: refer to the font size of the element itself
Media: visual
If the property is set on a block-level [p. 97] element whose content is
composed of inline-level [p. 98] elements, it specifies the minimal height of each
generated inline box.
If the property is set on an inline-level [p. 98] element, it specifies the exact
height of each box generated by the element. (Except for inline replaced [p. 30]
elements, where the height of the box is given by the ’height’ property.)
Values for this property have the following meanings:
normal
Tells user agents to set the computed value [p. 70] to a "reasonable" value
based on the font size of the element. The value has the same meaning as
142