You can target Outlook.com by using a CSS element called [owa] (Outlook Web Access), which is part of Outlook.com
HTML Email Knowledge Base
You can target AOL Mail by using the CSS selector class .aolmail_ and adding your styles.
Gmail webmail displays an image download icon when a user moves their cursor over an unlinked image.
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.