In IE8, the Microsoft introduced a new mode called compatibility mode which would render the page like IE7.
You can see this button next to address bar in stackoverflow.com. But when you access google.com / live.com, you would not see the compatibility mode button. How do we make sure the pages don't show this button the user?
Short answer:
Put this in your head tag to tell the browser that your page works in IE 8:
<meta http-equiv="X-UA-Compatible" content="IE=8" />
Also as per Jon Hadleys comment, to ensure the latest (not just IE8) rendering engine is used, you could use the following:
<meta http-equiv="X-UA-Compatible" content="IE=edge">