Collapsing Margin

8.3.1 Collapsing margins

In CSS, the adjoining margins of two or more boxes (which might or might not be siblings) can combine to form a single margin. Margins that combine this way are said to collapse, and the resulting combined margin is called a collapsed margin.

Adjoining vertical margins collapse [...]

Two margins are adjoining if and only if:

• both belong to in-flow block-level boxes that participate in the same block formatting context
• no line boxes, no clearance, no padding and no border separate them
• both belong to vertically-adjacent box edges, i.e. form one of the following pairs:
  • top margin of a box and top margin of its first in-flow child

Useful post:

http://stackoverflow.com/questions/9519841/why-does-this-css-margin-top-style-not-work