The page crashes when a WebKitCSSKeyframeRule is dynamically modified.
The bug is reproducable in Chrome 21.0.1180.57, and can be reproduced using the following code:

<style id="sheet">
@-webkit-keyframes test {
    from {}
}</style><script>
var keyFramesRule = document.getElementById('sheet').sheet.cssRules[0];
var keyFrameRule = keyFramesRule[0];
keyFrameRule.style.top = 0;</script>

(Demo at http://jsfiddle.net/LVpWS/68/).

This crash does not happen when the property already exists in the rule. Replace from {} with from{top:1px;} to verify: http://jsfiddle.net/LVpWS/69/