the framework's progressive enhancement support should include an API for clients to be able to register their own feature checking functions

https://github.com/fluid-project/infusion/blob/master/src/webapp/framework/enhancement/js/ProgressiveEnhancement.js

Current ProgressiveEnhancement implementation does not allow developers to add their own features or extend an existing feature check (fluid.browser)

Currently videoPlayer has a copy paste code taken from the ProgressiveEnhancement implementation in order to add check for HTML5 and browser full screen support.
https://github.com/fluid-project/videoPlayer/blob/master/js/VideoPlayer.js#L28-50

In this particular example a new set of features is being defined and then merged into staticEnvironment. The proper way is to reuse ProgressiveEnhancement.