Internet Explorer 8 : Will the existing sites break?
We do not have to follow the CSS 2.1 standard to be able to run it in IE 8. If your site runs on IE 7 without any issue the same will run on IE 8 with some simple effort. This has been described well in the KB Article # 952030.
In IIS you can add the following header info,
In the web.config under configuration section you can add add,
<add name="X-UA-Compatible" value="IE=EmulateIE7" />
You can modify a single page by adding meta tag,
<html xmlns="http://www.w3.org/1999/xhtml" >
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
<form id="form1" runat="server">
Find more on Microsoft's Interoperability Principles and IE8.