A week ago W3C published the first working draft of the W3C CSSOM View specification (written by Anne van Kesteren), and I must say I'm very happy with it. Since I was testing stuff anyway I created a new compatibility table for most of the methods and properties specified in this document, and browser compatibility is already excellent.
That's no coincidence. This specification contains definitions for many properties (and a few methods) that browsers have already been supporting for ages (such as offsetWidth), and W3C has paid scrupulous attention to the current implementation. No more theorizing into the blue — just check what browsers do and describe it in the specification. Excellent idea.