Main Content
With this layout you have the option of locating either the "Main Content" or the "Left Sidebar" first in the html source order.
The #inner div has a left margin set on it. That margin creates the space for the
#left div which is shifted into place with a negative margin.
The #main div is floated right and the #left div is floated left and
they are both nested in that #inner div.
By floating the #main div to the right it allows us to bring it first in the source
order. Then the #left div can be floated left which allows us to shift it directly into
the left margin of the #inner div with a negative margin on the #left div.
Float containment is done with a "clearfix" adaption using a pseudo :after block on the
#inner div for modern browsers and "haslayout" for IE 6&7.