Since web browsers have been around as long as the Web, it is possible to have customers viewing your web pages in browsers that are extremely old and missing features of more modern browsers. Progressive enhancement is a strategy of handling web page design for different browsers.
Progressive enhancement is a way of designing web pages so that the more features a user agent supports, the more features the web page will have. It is the opposite of the design strategy graceful degradation that builds pages for the most modern browsers first and then converts them to work with less functional browsers.
How to Use Progressive Enhancement
When you create a web design using progressive enhancement, the first thing you do is create a design that works for the lowest common denominator. At it's core, progressive enhancement says that your content should be available to all web browsers, not just a sub-set.
Design and layout is added in using external style sheets. And these styles are written to enhance the page as the user agents gain functionality.
Behaviors are added using external scripts. And these scripts are written to take advantage of browser support, but not deny access if that support is unavailable.