I thought this topic had been covered here before, but if you want to mimic frames, you'll either need some JavaScript to handle the process, or replace the content <div> with an <iframe> and set the 'target' attribute on the menu links to point to the <iframe>. And any links in the content pages will similarly need to be aware of their environment and have appropriate 'target' attributes or embedded JavaScript.
It would be far easier and better for the long-term performance and maintenance of your website to use a conventional webpage structure that has both the content and navigation links in the same document. There are many good reasons why frames have a bad reputation. If you want to have a single file to hold the navigation links, there are several methods available like PHP includes, Server Side Includes, or even JavaScript that will do the job without all of the encumbrances of frames or pseudo-frames.
Rick Trethewey
Rainbo Design
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