之前对html的video标签有过一阵子研究，主要用来判断视频文件在播放时事件触发的顺序。
罗列了下实际有用的事件属性：
loadstart: happened after load() method. The UA begins looking for media data.
durationchange: get the duration info of the media
loadedmetadata: The UA has determined the duration and dimesion(元数据包括：时长，尺寸以及文本轨道)
loadeddata: 当前帧数据可用，for the first time
progress: The UA is fetching media data
canplay: 可以播放当前及至少下一帧是可以播放的, readyState is newly to HAVE_Future_DATA
canplaythrough: readyState is newly to HAVE_ENOUGH_DATA
play: paused is newly false, after()
pause: paused is newly true
playing: playback is ready to start after having been paused or delayed due to lack of media data(readyState is equal to or greater than HAVE_FUTURE_DATA)
abort: UA stops fetching media data before it is completely download but not due to an error
waiting: Playback has stopped because the next frame is not available but UA expects that frame to become avaiable in due course
suspend: UA is intentionally not fetching media data currently
