Meta tags can be used to force Internet Explorer(IE) browser to use a specific standards mode. The X-UA-Compatible meta tag tells IE what view mode to use to render the current page.
Here is how to emulate different browser modes using meta tags:
Emulate IE 7:
<meta http-equiv="X-UA-Compatible" content="IE=7">
Emulate IE 8:
<meta http-equiv="X-UA-Compatible" content="IE=8">
Emulate IE 9:
<meta http-equiv="X-UA-Compatible" content="IE=9">
If the end client visits the page on a browser that is older than the specifed view mode (eg. User has IE 8 but view mode forces IE 9), the browser will ignore the meta tag and render the page the best way it can.