Actually Standard Definition NTSC is 720x486. 720x480 is the resolution of DV video, including DVD's but professional cameras and edit systems work at 720x486. When DV footage shot at 720x480 is incorporated into a standard def editing timeline you have to be sure to put video into the top and bottom three lines of video so it won't be rejected by network quality control, usually done by duplicating the top and bottom three lines of video. Viewers never see those lines since they fall out of the TV safe area at the edges of the screen. Since a digital video file for the net has no 'TV safe' area that gets lost in broadcast a lot of old broadcast editing tricks are having to be rethought. Conversely I've worked with editors who have learned the ropes editing footage for websites and the concept of a title safe area is absolutely foreign to them.