I am sure you have noticed Internet Explorer’s Compatibility View button, located on the address bar next to the ‘stop’ and ‘refresh’ buttons.
As we discussed in the blog Introducing Compatibility View, the Compatibility View button only appears when it makes sense to do so. For example, if the page’s Document Compatibility Mode is not set and if the page is trying to display in standards mode.
So, when a site uses the X-UA-Compatible <META> tag or HTTP header to set the Compatibility mode, IE8’s Compatibility View button should not display. However, we have uncovered a small problem where it still does display if the ultimate document mode of the page turns out to be Quirks Mode.
If you go to http://office.live.com/ you will notice this behavior. Here is also a simple page that demonstrates the problem:
<!-- Mimic Internet Explorer 7 -->
<title>My Web Page</title>
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
<p>Content goes here.</p>
Note: Having no DOCTYPE switches you to quirks mode.
Even though the Compatibility View button displays, the page functionality continues to work correctly. Therefore, the impact of this behavior is minimal.
The IE Support team