Key Concepts [Jan. 23, 2012, 11:50 p.m.]
Margin Collapsing
Top and bottom margins of blocks are sometimes combined (collapsed) into a single margin whose size is the largest of the margins combined into it, a behavior known as margin collapsing.
For example:
#section1 { margin: 1.5em 0 0; } /* top, right/left, bottom */
#section2 { margin: 2.5em 0 0; } /* top, right/left, bottom */
The winnning margin where the two sections separate is 2.5em because it is the largest value.