The Sharepoint designer 2010 workflow email body action is not a rich HTML, its not supported with this version but i found a quick way to add rich HTML in email body message.
Here is the steps:

• First create your HTML in dreamweaver or vs2010 or any other HTML editing tool
• Then open your workflow in SPD 2010
• Add “Send email” workflow action from ribbon
• Keep “Send Email” action selected/highlighted
• Then navigate to ribbon and select advanced properties
• Select last option Body
• Click on the button (...) to open the text pad
• paste your html including head,body and html.
• Click OK and open the action from workflow step
• Then add your lookups and test it.