With this layout you have the option of locating either the "Main Content" or the "Left Sidebar" first in the html source order.
#inner div is nested within the
#innerwrap div which has left and
right margins set on it. Those margins create the space for the left and right columns which are
shifted into place with negative margins.
#main div is floated right and the
#left div is floated left.
They are both nested in the
#inner div which is floated right to bring them first in
the html source order while allowing the
#right floated div to rise up to the same
vertical plane. The
#main div is floated right so the
#left div can be
floated left. That allows us to shift the
#left div directly into the left margin of
Float containment is done with a "clearfix" adaption using a pseudo :after block on the
#innerwrap div for modern browsers and "haslayout" for IE 6&7.