AJAX (Asynchronous Javascript and XML)
First, Javascript must be used to determine the browser.
The possibilities are:
The next step is to make a request to the server
Simple request example
Note the way that the browser address bar does not change as you navigate between the four pages on the site. This will make things load faster for the end user.
A big problem that I can see is that most of these pages will now become 'invisible' to a search engine, or a least scanned in an isolated format and therefore do no favours for your SEO.
By implementing this method of splitting the navigation and content for a simple site, I can see that it would be easier for the novice to update any content as they only need to touch the simple HTML content files and not get put off by the javascript end of things.