Have you ever thought of reversing your workflow? This should ensure better compatibility especially with regards to CSS.
One of the things I've noticed is that if you target IE9 first, you would notice that it is easy to scale up from that to better browsers. Think of IE9 as the minimum experience baseline. What of everything fancy is stripped leaving you with a basic but functional site?
Also, this will also help to ensure that you don't forget about non-WebKit. Keep in mind that code for IE10 will usually almost always work in FF16+ and Opera 12.5 and newer without any modifications.
My main concern is always try to develop with the standard API whenever it's possible, so most browsers will just work (with very few adjustments).
For me there's two problems targeting IE9 first, one is that i work on Ubuntu, so while it's virtually possible, it's a pain to do so, second and most important, Chrome Developer Tools are amazing, and it doesn't seem that IE9 developer tools have evolved much since the poor IE7 ones.
So i just start with the best experience possible (on Chrome and testing casually on Firefox), while trying to make CSS just gracefully degrade (animations, round corners and other stuff usually degrade nice).
When both Chrome e Firefox work, Opera takes very little time. The same should hold true to IE10, i'm not sure, as i never tested on it (it has such a low marketshare ATM that i do not worry, yet). I just tell my users to use Chrome Frame, and when i can (or need) i will support IE10, and eventually IE9..
If you look at this page:
http://yuilibrary.com/yui/environments/ is easy to understand my main reason for doing this way. Both Chrome and Firefox are not treated as version, we can just assume it will work in any version (and that the user will eventually be in the newest version), where even if i ever support IE10, it's not guaranteed that it will work in IE11, and that's my main problem, because IE11 should be here by next year, so i will need to test everything for it again? I don't have time for that if i want to develop new things.
Maybe i'm wrong and IE10-11 will just work with standards, then i think i will finally support it.