Without even going into HTML5 or Javascript, many video formats are streaming formats that are designed to be played forward. Playing it backwards would require decoding the whole stream, storing each raw frame on the disk to avoid clobbering memory, then rendering the frames backwards.
At least one person actually tried that using mplayer, though, so it can be done, at least in principle.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
