title: RequiredFieldValidator sometimes does not work, but unable to reproduce it
body: <p>I am debugging an APS.Net Webform application that sometimes accepts an empty value from a textbox field.  It seems that the RequiredFieldValidator does not working. In web log, all incidences seemed to be related to IE 11.0 browser, no minor version data available. The jQuery file used is jquery-1.7.1.min.js,  httpRuntime targetFramework is 4.5, and IIS app pool is ASP.NET v4.0. Also, the validation setting used is </p>
  
  <pre><code>&lt;add key="ValidationSettings:UnobtrusiveValidationMode" value="None" lockItem="false" /&gt;
  </code></pre>
  
  <p>I read some posts online, and some of them pointed to .Net framework version and jQuery validation in IE 11 browser. However, I was unable to reproduce the issue using IE 11.0 updated version 11.0.105(KB4480965). See code below.</p>
  
  <pre><code>&lt;span class="Bold"&gt;Item ID&lt;/span&gt;
  &lt;asp:TextBox ID="txtItemID" runat="server"&gt;&lt;/asp:TextBox&gt;
  &lt;asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="txtItemID" ErrorMessage="Error" InitialValue="" SetFocusOnError="true" Display="Dynamic" ValidationGroup="Add"&gt;&lt;/asp:RequiredFieldValidator&gt;
  &lt;asp:Button ID="btnAdd" runat="server" OnClick="btnAdd_Click" Text="Add an Item" ValidationGroup="Add" /&gt;
  </code></pre>