At a high level, after the 'play' event has occurred, you will need to
bind additional code to monitor the progress of the video with something
like setInterval. Within that function, you'd need to compare the
"duration" of the video object to the "currentTime" in order to figure
out if you had reached one of your milestones. You'd do some division of
the two, currentTime/duration *100 to get a percentage to compare
against and fire off a tag when you meet the milestone criteria.