Based on the things that are suggested on http://nicolasgallagher.com/css-image-replacement-with-pseudo-elements/
I found a solution which should work in all (modern) browsers except IE7 for which would need a fallback to the old solution. On http://paste.xinu.at/qmy/
you can see a diff of the current state. I haven't done all buttons yet, the different icons for the different modes are missing and I haven't done anything for rtl support. None of these open points should be a problem though. I also haven't done anything for IE7 support, with these changes the icons will be missing in IE7 (as if images were disabled I think). Of course one could also improve the styling of the text when images are disabled.
[Edit] Just tested in IE7, somehow resetting the width to auto doesn't work in IE7. I fixed it by applying a large-enough min-width however. Unfortunately that might not work for all languages, no idea how to solve that properly. In http://paste.xinu.at/Driac3/
this ugly fix and restoring the original behavior in IE7 is included.