Let's say that you're using CSS to layout your page. It is possible to place your navigation after the content. You can acheive this by floating your navigation and content containers.
Traditionally the navigation comes first in the actual HTML. To make this type of design more accessible, it is common to see a link which reads "Skip Navigation" that will take you straight to the content. Usually this is the first element on the page which gives the user the option to skip all of the header and navigational elements.
Consider the following example...
HTML Code:
<body>
<a href="#content">Skip Navigation</a>
<div id="header">
<h1>Your header</h1>
</div>
<div id="nav>
<ul>
<li><a href="#">Links Here</a></li>
<li><a href="#">Links Here</a></li>
<li><a href="#">Links Here</a></li>
</ul>
</div>
<div id="content">
<h2>Your content</h2>
<p>Yay for content!</p>
</div>
</body>
This will allow the person to 'skip' the header and navigation and go straight to your content.
Assuming your header and navigation is light weight and simple (e.g. not a mess of javascript, place holder images, image maps etc) you'll be fine from an accessibility standpoint whether your put the content before or after the navigation.
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