Viewability

An ad impression counts as soon as the creative has loaded, even far below the visible area. Viewability corrects that: under the Media Rating Council standard a display ad counts as viewable when at least 50 percent of its pixels were in the viewport for one second, and 30 percent is enough for large sizes.

For advertisers it is the basis for buying on vCPM. For design it means the message has to be there in the first second. An animation that reveals logo and headline only after three seconds wastes viewable impressions, so good effects start with the essentials and end on a complete end frame.