A) Element float:left;
margin:0;
B) Normal
top-margin:50px;
Additional margin above element A can be seen in Firefox. Should look identical to version 3.
margin:0;
top-margin:50px;
Removing the margin on element B removes both margins, proving the margin is duplicated.
margin:0;
margin:0;
Setting the padding to .main-content fixes the layout (but is only possible if you want the padding).
margin:0;
margin:0;