HTML Parsing and Rendering in Outlook 2007.
Microsoft Office Outlook 2007 uses the HTML parsing and rendering engine from Microsoft Office Word 2007 to display HTML message bodies. The same HTML and cascading style sheets (CSS) support available in Word 2007 is available in Outlook 2007.
This article provides reference documentation related to supported and unsupported HTML elements, attributes, and cascading style sheets properties.
http://msdn.microsoft.com/en-us/library/aa338201.aspx