After using CSS to animate a value by vu's, the size becomes static and does not respond to window resizes. I have a codepen example at: http://codepen.io/anon/pen/gbyBVR Comparatively tested with Firefox 37.0 and IE11(.0.9600.17690), both of which continue to respond as expected. Webkit-based browsers (Chrome 41.0.2272.118 m (64-bit), Vivaldi 1.0.118.19, Opera 28.0.1750.48) lock the size once animating begins.
I can reproduce in Safari 8.0.4.
Safari Technology Preview 167 behaves the same as Chrome and Firefox. This has been fixed. Please reopen if you find that it is otherwise.