How compatible are new unity web apps with older versions of the unity web player?

I generally prefer to use the most recent version of the Unity editor for development. My concern is that I cannot guarantee the web player version that my customers are using.

I just tried testing my unity app using web player 3.5 (with the app itself built with the latest 4.3.4) and was surprised that it worked great. So apparently old versions of the web player can still play content created with new versions of the unity editor? How does this work and how far back can we safely assume this will work? Is it dependent on what features I use in my game?