I had a bug happen when switching web sites using the animation in ajax. When this animation triggers there is a white background for the first post back. Subsequent post backs trigger as transparent as one would expect.
<AjaxControlToolkit:UpdatePanelAnimationExtender TargetControlID="UpdatePanel1" ID="AnimationExtender1" runat="server"><Animations><OnUpdated><sequence AnimationTarget="mydiv"><Pulse Duration=".1" /><ScriptAction script="changeOpacityDiv();" ></ScriptAction></sequence></OnUpdated></Animations></AjaxControlToolkit:UpdatePanelAnimationExtender>
This only happens when the background of the page is a repeatable image.
By setting a background image (in my case a blank or empty image ) to 'mydiv' fixes this issue. It was hard to catch which is why I am posting it.
Really annoying the first post back flashes a white background. Hope that helps someone else.
Also, this may be only an IE 10 issue since firefox didn't seem to have an issue.