The HTML online validator can't tell where the arror ocurred exactly. Tries to narrow the posibility by selecting the first element the error triggered and the last one suffering it. In this case,...

Some little dumb mistake I'm sure...
p.s.: check the little thingy in red inside your code, That's what's missing here.

So you want them to be looking something like this (see attachment image)?
Add this line in your
CSS
#main img {width:100%}

Delete everything marked RED

This code enables you to publish a CSS only with mouse click and keyboard tab image swap.

Attached is a Demo example of how it would look on IE6.

To ensure the greatest possible cross-browser...

Yeah, it looks as it should on IE.

All container elements are broken in Netscape based browsers.
This is known as a bleeding fox syndrome bug, or "fox on fire behaviour".
It will drop...

Here,

First thing you should do is to remove your script from HTML element and place it somewhere before HEADER element closure.

Second:
Something on the right should follow something on its...

stay away from that site and block them.

few notes on interclick.com
http://google.com/safebrowsing/diagnostic?site=interclick.com/

i suggest you block these addresses:
www.interclick.com...

that's because you are doing it the wrong way, -it is the div2 who should have its margins set to auto and there should be no float.
-Here, try this:

.DIV1 {
width: 100%;
height: 70px;
}...

try this site (http://layouts.ironmyers.com, might help you alot.