1) With Margins

Additional margin above element A can be seen in Firefox. Should look identical to version 3.

A) Element float:left;

margin:0;

B) Normal

top-margin:50px;

2) Without Margins

Removing the margin on element B removes both margins, proving the margin is duplicated.

A) Element float:left;

margin:0;

B) Normal

margin:0;

3) With Margins with padding

Setting the padding to .main-content fixes the layout (but is only possible if you want the padding).

A) Element float:left;

margin:0;

B) Normal

margin:0;