From IE8 Help:
Why don’t some websites display correctly in Internet Explorer 8?
Websites that were designed for earlier versions of Internet Explorer might not display correctly in the current version. However, you can often improve how a website will look in Internet Explorer by using a new feature called Compatibility View.
When you turn on Compatibility View, the webpage you’re viewing—as well as any other webpages within the website’s domain—will be displayed as if you were using an earlier version of Internet Explorer.
To turn on Internet Explorer Compatibility View
Open Internet Explorer.
Click the Tools button, and then click Compatibility View.
If Internet Explorer recognizes a webpage that is not compatible, you will see the Compatibility View button on the Address bar. To turn Compatibility View on or off, click the Compatiblity View button .
From now on, whenever you visit this website, it will be displayed in Compatibility View. However, if the website receives updates to display correctly in the current version of Internet Explorer, Compatibility View will automatically turn off.
Notes
Not all website display problems are caused by browser incompatibility. Interrupted Internet connections, heavy traffic, or website bugs can also affect how a webpage is displayed.
Some websites hard code in IE7, some IE8, etc. So use of Compatibility View may or may not work. IE8 causes CSS issues and broke expression use in scripts. Some issues could be due to the user blocking scriplets. Bottom line, there is no simple answer.