Remove error class when validator callout is closed

I have several validator callouts, they all look around the same.

<ajaxToolkit:ValidatorCalloutExtender ID="vceLastName" HighlightCssClass="not-valid" TargetControlID="rfvLastName" runat="server">

As you can see, I have a highlightcssclass which is added whenever the input is invalid.  Is there anyway I can remove that class when the validor callout is closed using the 'x' in the corner of the callout?

Is there anyway I can just tie into the close event of the callout?

Thanks