Hello again PonchoX,
I did not get to spend much time on this before...
That mystery gap is caused by the default 16px top/bottom margin on your ul inside #mainNav at the top of section#mainContent.
To illustrate, make your CSS look like this -
Code:
nav#mainNav ul {
text-align: center;
border-bottom: 2px #699 solid;
margin: 0;
}
You can see that zeroing out the margin makes that gap go away. I think for that to work better you should have a look at how
collapsing margins work. That way you'll be able to keep that top/bottom margin.
#mainContent {overflow: auto;} makes the columns and #footer meet but your
box model issues give you both X and Y scrollbars that I'm sure you'd rather not have.
The border method of curing collapsing margins might work better for you. Try adding
#mainContent {border-top: 1px solid #000;} to your CSS.