<https://w3schools.invisionzone.com/topic/53171-problem-with-blur/#comment-293028>
<http://schema.org/text>
"Some advertisers used to make an ad window pop up and then hid it behind the main window. It was called \"pop under\" and browsers probably changed to prevent that from happening. "
<https://w3schools.invisionzone.com/topic/53171-problem-with-blur/?tab=comments#comment-293006>