Still in the early stages of design/development. However, here is an early, working prototype, implemented in WebKit as described here: http://www.xanthir.com/blog/ under "aspect-ratio CSS property".
Created attachment 70894 [details]
Experimental Patch For aspect-ratio property
Stable permalink for the spec: http://www.xanthir.com/blog/b4810
Created attachment 75503 [details]
Experimental patch rev. 2 for aspect-ratio property
FWIW, this has already been discussed at www-style http://lists.w3.org/Archives/Public/www-style/2010Oct/0046.html and http://lists.w3.org/Archives/Public/www-style/2011Oct/0550.html. It's just blocked on Tab and Fantasai having enough time to put in into a formal spec as far as I can tell.
I don't see any problems with moving forward with this as a webkit-prefixed property.
I'll use this master bug to track the new aspect-ratio work: