Most people talk about progressive enhancement right now as serving browsers with javascript (enhanced version), and browsers without javascript (simple version).
But there is such a big difference in javascript performance between browsers that it may be useful to apply that term to support for choosing between javascript based features between browsers.
In a complex web app with numerous non-absolutely essential features (and animations), is it worthwhile to start thinking about cordoning them off for say, these sets of features should work in all browsers, and these sets of features only in Chrome and Safari, and these in Firefox and Chrome and Safari and Opera, and so on, because enabling certain features in certain browsers would be too slow.
Sometimes I feel like the user experience would improve if they did not have access to certain non-essential features. For instance disallowing IE users from resizing certain panels that Chrome users would be able to resize.