All of a sudden I started to find the following line in most of the web pages I was browsing, including ones I made where I'm sure I didn't place that code:
<div id="myEventWatcherDiv" style="display:none;"></div>
I googled and binged and yahooed the whole interwebs but couldn't find any explanation. Then I searched my browser plugins directory and found the culprit:
wpa.safariextension which is part of the DivX Inc. plugins galore installed with DivX for Mac.
Since these DivX plugins do not bring any value, but on the contrary bring very bad memories from the RealPlayer era of shoddy tactics (“Click here to get a better video experience" my ass!), you can safely trash them and be done with this cruft.