What you are describing would be characterized by unobtrusive javascript.
see; http://en.wikipedia.org/wiki/Unobtrusive_JavaScript
You write your html in the most semantic SEO friendly way possible for search engines and users with javascript turned off, then add your script separately to add your bells and whistles.
A framework such as jQuery is often useful.
For example;
<a href="/about" id="about">About</a>
could be given another function via an external javascript file containing;
$("#about").click( function() {
//fancy code here
return false;
});
which would stop the user being taken to /about and execute the given javascript instead.
Essentially this is the inverse of your suggestion; rather javascript is only used if it's available to enhance the existing html.