Gmail webmail displays an image download icon when a user moves their cursor over an unlinked image.
HTML Email Knowledge Base
The IE=edge meta tag, placed within the <head></head> of your HTML email, enables CSS3 and media query support on Windows Phones, version 7.5 and higher.
By default AOL uses it’s own font styling. The fix is very simple and can be overwritten using your own embedded CSS.
If you have set an align attribute in an image to right or left, Thunderbird will add a 3 pixel margin these images.
The accessibility for HTML emails is often overseen, however, with a few tweaks can greatly improve a readers experience using assistive technology.