Dreamweaver's design view is not a web browser. Sometimes, the design view in Dreamweaver just doesn't properly render the page.
A couple of possible causes are:
-A tag that is not closed properly.
-Dreamweaver's visual aids. you can disable them by going to View -> Visual Aids and checking Disable all.
there are a couple problems i see in the code for you page that may be causing this:
1) There is template code on the page, but the page is not derived from a template. for example:
<!-- TemplateBeginEditable name="panier" -->....<!-- TemplateEndEditable -->
2) The page does not have a proper doctype tag, this will cause display issues in some browsers.
add the following just before the opening <head> tag:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
see the following thread for details on the headers already sent error: