Do you test HTML designs in Webmail using both Firefox and IT browsers? It's not clear from the service description.
I've looked only at Gmail in the two browsers, and there appears to be some differences in how they display and manage images. I assume other Webmail services also render a bit differently.
Also, I'm a little concerned that my CSS isn't registering in either browser, although the CSS and HTML have been validated. Will the display test results include any tips on why my design is breaking?
For the most details on which email clients support which CSS see http://www.campaignmonitor.com/css
The testing service only shows results from Internet Explorer at the moment though.