You might already have experienced something -- that your site displayed perfectly on Internet Explorer (IE) 6 but broke down in IE7. Now, IE8 beta 1 
is out! More trouble is on its way. But, there is hope...or is there?
The Header
IE8 comes with a new feature, "Rendering Mode Switch". Basically, just putting the following tag into the head section of your page makes IE8 display it as IE7 (IE8 uses the IE7 rendering engine). So, there shouldn't be any compatibility issues.
<meta http-equiv="X-UA-Compatible" content="IE=7" />
Make sure to put the above tags first in the head section (immediately after the <head> tag).
Also, there are some extensions, like:
<meta http-equiv="X-UA-Compatible" content="IE=8;FF=3;OtherUA=4" />
"FF" stands for Firefox, "OtherUA" stands for other user agent (other browser). But these extensions are useless. IE8 cannot use another browser's (i.e. Firefox's) rendering engine because IE8 doesn't have it. And, Microsoft would never put another browser's rendering engine into IE8.
Concerns
Before adding the X-UA-Compatible header, consider the following:
- This is Microsoft's standard which other browser (i.e. Firefox) developers will never adapt.
- Some people are reporting that the X-UA-Compatible header is causing problems on other browsers.
- The X-UA-Compatible header is not working well even on IE8.
- You can't know whether or not Microsoft will keep the X-UA-Compatible feature in future browsers, such as IE9.
My Opinion
IE is still the most used web browser. It's hard to ignore the X-UA-Compatible header. But, IE is losing the browser market share to Firefox (and IE is just a copycat version of Netscape). Sticking with W3C's 
standards would save you from trouble in the future.
© May, 2008