Windows® Internet Explorer® 8 is Microsoft’s latest web browser. Unlike previous versions, Internet Explorer 8 renders content in the most standards-compliant way possible. This means that web pages will be displayed in Internet Explorer 8’s standards mode by default. This decision was driven by Microsoft’s Interoperability Principles and our goal of helping developers in the long-run, by enabling them to write one set of code to run on all platforms.
By focusing on web standards, we ultimately want to help the web move forward. However, we understand that in the short-run, sites that rely on the behavior of previous versions of Internet Explorer may not display correctly in Internet Explorer 8. We recognize that it takes time for organizations to update their sites for Internet Explorer’s transition to web standards. To bridge this transition and allow you additional time to update and test your web properties with Internet Explorer 8, we're offering a short-term solution: the inclusion of your site on the Compatibility View list.
Through product feedback channels, our users have indicated that, during the beta period, your website may not have been compatible with Internet Explorer 8 in its default, standards-based mode. As a result, your domain has been added to a list of sites that, for the short-term, is most likely to be displayed better in Compatibility View. All Internet Explorer 8 users are given the choice to use this list, and the subset that chooses to do so will see your domain automatically displayed in Compatibility View, without additional user interaction or notice. Your site has high traffic volume (in your region) and having a compatible website ensures a significant number of Internet Explorer 8 users will have a great experience.
This list will be periodically updated and automatically downloaded to Internet Explorer 8 users who’ve opted-in to use Compatibility View updates from Microsoft. For more information on Compatibility View list updates, please see – http://go.microsoft.com/fwlink/?LinkID=139381
Preferred: Revise site content to support the standards-compliant default view for web pages in Internet Explorer 8.
Internet Explorer 8 supports the HTML 4.01 Specification more closely than any previous version and supports some features of the HTML 5 Specification. This release of Internet Explorer also offers full support for CSS 2.1 and supports some popular features of CSS 3.0.
For more information, please see http://go.microsoft.com/fwlink/?LinkID=158309
You can see if your site is on the list:
http://www.microsoft.com/downloads/details.aspx?FamilyID=b885e621-91b7-432d-8175-a745b87d2588&displaylang=en (.xls file)
What does this all mean start writing you web site using web standards
You can check your site here: http://validator.w3.org/
Some other tools