Where is the "aspNetDisabled" class defined and why does ASP.NET render an interfering duplicate CSS "class" attribute for it?

For anyone that might still be looking for this, we can define this css class during Application_Start in the Global.asax:

void Application_Start(object sender, EventArgs e)
{
    WebControl.DisabledCssClass = "customDisabledClassName";
}

Source: WebControl.DisabledCssClass Property (MSDN)

I ended up doing the following, which effectively removes the insertion of the extra class for disabled items.

void Application_Start(object sender, EventArgs e)
{
    // Code that runs on application startup
    WebControl.DisabledCssClass = "";
}

You might want to look at this:

http://msdn.microsoft.com/en-us/library/system.web.ui.htmlcontrols.htmltextarea.aspx

For one, there is no "class" attribute. This is a HTML control; if you want server-side access, you need to add the runat="server" attribute. There is a "Disabled" property. There is also a "Style" property.

Can you explain exactly what it is you are trying to do and why you're not using a TextBox instead with the TextMode property set to multiline?