title: Where is the "aspNetDisabled" class defined and why does ASP.NET render an interfering duplicate CSS "class" attribute for it?
body: <p>When I set the "Disabled" property of an ASP.NET TextBox control to false, the final rendered HTML textarea tag (sent to the browser) includes an 'class="aspNetDisabled"' attribute in addition to the 'disabled="disabled"' attribute.  Where is the "aspNetDisabled" class defined?</p>
  
  <p>It seems to me that it's not defined anywhere, and the real killer is that this useless class is interfering with my defined classes, because ASP.NET is rendering this into the control as a duplicate CSS class attribute:</p>
  
  <pre><code>&lt;textarea [...] disabled="disabled" class="aspNetDisabled" class="boxsizingBorder largeinput"&gt;
  </code></pre>
  
  <p>Can anyone else confirm this bug?</p>
  
  <hr>
  
  <h2>Additional Info</h2>
  
  <p><strong>IIS Version:</strong> 7.0.6000.16386<br>
  <strong>AppPool .NET Framework Version:</strong> v4.0<br>
  Server control tag in ASPX page:</p>
  
  <pre><code>&lt;asp:TextBox ID="txtInput1" class="boxsizingBorder largeinput" runat="server" TextMode="MultiLine"&gt;&lt;/asp:TextBox&gt;.
  </code></pre>