I use the following criteria:
Each browser may choose to include different features of webkit in their final versions, so they are not always the same.
The conclusion will change when hiding certain browsers/eras, which is why it's there.
The "far past" is based roughly on the oldest version of a browser still in use. See for example YUI's graded browser support chart
The "future" eras are trickier, since it's not always obvious when a browser will be released. But in most cases you can make a pretty good guess.
When its primary purpose is largely fulfilled. It does not mean it's 100% supported, just that it's usable in most cases
I will admit that part of my reason to create this page was to highlight how many features could be used already if it weren't for IE holding us back. However, I have tried to keep things as balanced as possible and no intentional bias should exist.
filter, CSS behaviors, etc?
As mentioned as a criteria, it needs to be likely for most if not all browsers to support a feature, ideally through standardization. Many IE-specific features were never proposed to the W3C/WhatWG, and other browsers have shown little to no interest in adopting them.
If it turns out that other browser-specific features show little chance of reaching further adoption, they will be removed them as well.