I am using ajaxcontrols and in that all controls are working well But I had not found any of the control named as HtmlEditorExtender so I had just copied this tag in .html page 

<%@ Register TagPrefix="ajaxToolkit" Namespace="AjaxControlToolkit" Assembly="AjaxControlToolkit"%>
<ajaxToolkit:ToolkitScriptManager ID="ToolkitScriptManager1" runat="Server" />
<ajaxToolkit:HtmlEditorExtender TargetControlID="txtComments" runat="server" />
<asp:TextBox ID="txtComments" runat="server"></asp:TextBox>

here is working fine but the problem arises on , then i downloaded Nuget.tool on reading some of the problems facing just like me but still its not working 

what shall I do? 

My Web.config file is 

  <configuration>
<system.web>
<compilation debug="true" targetFramework="4.0"/>
<pages>
  <controls>
    <add tagPrefix="ajaxToolkit" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" />
  </controls>
  <sanitizer defaultProvider="AntiXssSanitizerProvider">
    <providers>
      <add name="AntiXssSanitizerProvider" type="AjaxControlToolkit.Sanitizer.HtmlAgilityPackSanitizerProvider" />
    </providers>
  </sanitizer>
</pages>
</system.web>
</configuration>