We often get emails and phone calls from customers unable to explain why their CSS round corners, box-shadows, RGBA transparencies, etc. look perfect on their computer - while clients are reporting that these CSS techniques do not work in IE9 on their computers. How could this be? In 2 words: Compatibility View. Many people accidentally click the little CV icon at the right edge of the address bar. IE9 goes into Compatibility Mode and displays pages as if it were IE7. No round corners, no shadows - you get the picture.
You can force IE9 to stay in Standards Mode. In fact, you can make the CV icon disappear for your pages. All it takes is this little Meta tag in the head of your page:
HTML version:
<meta http-equiv="X-UA-Compatible" content="IE=Edge">
xhtml version:
<meta http-equiv="X-UA-Compatible" content="IE=Edge"/>
That's it.
IE9 (and IE8 too) will also lock in their own Standards Mode.
Note: If you are using an HTML5 DOCTYPE and are concerned (you should not be) with perfect validation, you can simply add the meta tag inside a generic IE conditional comment:
<!--[if IE]>
<meta http-equiv="X-UA-Compatible" content="IE=Edge">
<![endif]-->
