You need to be wary of XSS when doing stuff like this:
document.getElementById('<%= Label1.ClientID %>').style.display
The chances are that no-one will be able to tamper with the ClientID of Label1 in this instance, but just to be on the safe side you might want pass it's value through one of the AntiXss library's methods:
document.getElementById('<%= AntiXss.JavaScriptEncode(Label1.ClientID) %>').style.display