The latest incarnation of Internet Explorer 8 adds a new twist to standards compliant web pages. When viewing a webpage in standards mode the user is presented with a compatibility icon that can be toggled to view the page using the Internet Explorer 7 engine.
However, as web master you have ultimate control. By using the X-UA-Compatible meta tag in your documents you can choose which mode you want your page rendered in and hide the button from users.
<meta http-equiv="X-UA-Compatible" content="IE=Edge" />
Always uses the latest available rendering engine.
<meta http-equiv="X-UA-Compatible" content="IE=8" />
Display using the IE8 rendering engine.
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
Display standards mode documents in IE7 standards mode, quirks mode documents in IE7 quirks mode.
<meta http-equiv="X-UA-Compatible" content="IE=7" />
Use IE7 standards mode.