I believe he was referring to web fonts [1], which, like images, can be configured to be downloaded from the sender's server, thus informing them that you opene

I believe he was referring to web fonts [1], which, like images, can be configured to be downloaded from the sender's server, thus informing them that you opened the e-mail. [1] http://www.html5rocks.com/en/tutorials/webfonts/quick/ http://www.html5rocks.com/en/tutorials/webfonts/quick/