1 of 1 people found this helpful
That litmus email, looks to be using some CSS wizardry that overwrites the content properties of some containers:
<!-- LIVE DYNAMIC TWEETS IN AN EMAIL AWW YISSS -->
<!-- TWEET #TEDC15 TO SEE YOUR TWEET APPEAR IN THE STREAM! -->
<!-- THIS ENTIRE SECTION SHOWS FOR WEBKIT ONLY, USES DYNAMIC CSS FILE THAT OVERWRITES THE CONTENT PROPERTY FOR EACH EMPTY TWEET DIV - REFER TO THE LINKED STYLESHEET IN THE HEAD TO SEE THE CSS FOR THIS SECTION -->
<!--[if (gte mso 9)|(IE)]>
<div style="font-size: 0px; height: 0px; width: 0px; max-height: 0px; max-width: 0px; line-height: 0px; opacity: 0; overflow: hidden; display: none; mso-hide: all;"><![endif]-->
Pretty clever, but as noted, it will only work for emails loaded in a webkit browser, so something like Gmail within Chrome or Safari.