jquery css issue on page partial postback

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CSS

jQuery

I have a custom jquery css applied to from buttons but it looses its styling on partial page postback. After investigating I foundout that IE is removing css classes on partial postback.

<asp:Button ID="btnSerach" CssClass="button" runat="server" Text="Find Customer" OnClientClick= " return open_search_box()" />

When the page renders it first time in IE it looks like

<input name="ctl00$JobLogPageContentPlaceHolder$btnSerach" class="button ui-button ui-widget ui-state-default ui-corner-all" id="ctl00_JobLogPageContentPlaceHolder_btnSerach" role="button" aria-disabled="false" onclick=" return open_search_box();" type="submit" jQuery1332289350068="6" value="Find Customer"/>

After partial postback it becomes :

<input name="ctl00$JobLogPageContentPlaceHolder$btnSerach" class="ui-button" id="ctl00_JobLogPageContentPlaceHolder_btnSerach" onclick=" return open_search_box();" type="submit" value="Find Customer"/>

Note the class attribute in both cases. Why IE is doing this ?