Why does IE9 give the option to render in compatibility mode

Question

Why on some sites is there the option to render the page in compatibility mode, as in the little broken page icon in the address bar. Then on other sites it doesn't.

What ie9 logic in deciding if this option should be present or not

Even if the site is html5 and if i've added

<meta http-equiv="X-UA-Compatible" content="IE=9">

Answer 1

Ok finally managed to get rid of compatibility mode icon from IE9. Id tried putting the meta tag in various position within the code. But always seemed to have the option to run in compatibility mode.

But adding the following to the .htaccess file soloved it for me.

Header set X-UA-Compatible "IE=edge"

Answer 2

I would recommend making sure you have a recent doctype set to help IE decide which renderer to use. If you wish to override it's built-in logic you can also set the render mode for IE9 using this meta tag. <meta http-equiv="X-UA-Compatible" content="IE=edge">

IE9 uses a variety of methods for determining if your page may have compatibility issues. Usually this involves the use of methods or page components that have changed behavior in later releases. Some examples, such as conditional comments, are mentioned in the msdn article found here.