I believe he was referring to web fonts [1], which, like images, can be configured to be downloaded from the sender&#x27;s server, thus informing them that you opened the e-mail.<p>[1] <a href="http:&#x2F;&#x2F;www.html5rocks.com&#x2F;en&#x2F;tutorials&#x2F;webfonts&#x2F;quick&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.html5rocks.com&#x2F;en&#x2F;tutorials&#x2F;webfonts&#x2F;quick&#x2F;</a>