Progressive enhancement is a strategy for web development which ensures that no matter its technical setup, any visitor to the website will always have access to the core information / functionalities, whilst users with a recent browser version, a specific plugin (like Adobe Flash player or Blaxxun VRML ) will get the richest user experience (like using ajax for a more reactive interface, etc.).
Its main principles are: accessibility, semantic HTML markup, and external stylesheet and scripting technologies (separation between content, styling and behaviour).