The good news is that at least now a large majority of users are on browsers with auto update. So once the spec is finalized and browsers start to enable it by

The good news is that at least now a large majority of users are on browsers with auto update. So once the spec is finalized and browsers start to enable it by default you should get a pretty large % of users who will support it. You will likely have to have a separate stylesheet for older versions of IE (and possibly other browsers) but honestly you likely have that anyway. And it's a lot easier to update a few values in that then throughout your entire site. We'll see how it plays out but I certainly wouldn't expect to use them in production for quite a while yet. I'm sure that is why they hid it behind an about:config option for now.