Some websites render content differently according to the User Agent that is being sent. For example, if the User Agent is IE 6, the language used to render the webpage (e.g. PHP) could be used to add a class "ie6" to an element, and then CSS is used to control what gets displayed or not. Or, a separate js file can be added depending on the user agent.
Some years ago, Opera recommended that you set the User Agent as IE 6, because websites will consistently not render if the User Agent was not any of IE or Mozilla. But, I think this is no longer the case.