margin: 0 0 50px 0;
margin: 20px 0 0 0;
//common sense would seem to suggest that the vertical margin between
// the <h1> and the <h2> would be a total of 70px (50px + 20px);
//but due to margin collapse, the actual margin ends up being 50px
Top and bottom margins of elements are sometimes collapsed into a single margin that is equal to the largest of the two margins.
This does not happen on left and right margins! Only top and bottom margins!.