[cssom-view] behavior of MouseEvent.offsetX/Y underspecified

Two issues:

1) Need to clarify how "the origin of the padding edge of the target node"
behaves when the target node has zero or more than one CSS fragment. I
suggest using the first fragment, and returning 0 if the target node has no
CSS fragments.

2) How does this behave if element geometry changes between the event being
dispatched and "offsetX/Y" being accessed? For example, given
<div id="d" style="position:absolute; top:100px; left:100px; width:100px;
height:100px; background:yellow"></div>
<script>
d.onmousedown = function(e) {
  alert(e.offsetX + "," + e.clientX);
  d.style.top = d.style.left = "0";
  alert(e.offsetX + "," + e.clientX);
};
</script>
In Chrome, clicking in the middle of the "d" element, I observe two alerts
showing "50,150". But, if I comment out the first alert, I get an alert
showing "150,150"! So it looks like in Chrome, the first time you call
offsetX/Y it computes the offset point using the current geometry, stashes
it in the event object, and returns it unchanged afterward. This doesn't
sound like something we should spec. I'd prefer to say that it's always
computed using whatever the current CSS geometry happens to be.

Rob
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