function SelectDiv() { document.getElementById("modalsuccess").style.display="block"; }
modalsuccess.Style["display"] = "block";
<div id="modalsuccess" runat="server"> // Your content </div>
modalsuccess.Visible = true;
modalsuccess.style["display"]="block";
Page.RegisterClientScriptBlock(this().GetType(), "ShowDiv();");
<div id="modalsuccess" runat="server" visible="false" > // Your content </div>
protected void Button1_Click(object sender, EventArgs e) { modalsuccess.Visible = true; }
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