<button onclick="getVideoTracks()" type="button">获得可用视频轨道的数量</button>
<video id="video1" controls="controls">
<source src="/example/html5/mov_bbb.mp4" type="video/mp4">
<source src="/example/html5/mov_bbb.ogg" type="video/ogg">
Your browser does not support HTML5 video.
myVid=document.getElementById("video1");
function getVideoTracks()
alert(myVid.videoTracks.length);