Do you have the same problem in Firefox and do you have the “Video DownloadHelper” AddOn installed?
To make it short:
– I tested in IE, Chrome and Firefox.
– Problem only occured in Firefox.
– Started Firefox in safe mode via SHIFT and doubleclicking the icon.
– The random div disappeared.
– Deactivated my Firefox plugins one by one.
-Problem was gone by deactivating the Video Downloadhelper.
Even the random div is called vdh-mask which further solidifies my assumption.
the Random div looked like this:
<div class="vdh-mask vdh-1937360175" style="width:1903px;height:1070px;top:40px;left:0px"></div>