var canvas = document.createElement("canvas");var video = document.querySelector("video");var ctx = canvas.getContext("2d");var newWin = window.open();
canvas.width = video.videoWidth;canvas.height = video.videoHeight;
ctx.drawImage(video, 0, 0);
newWin.document.write("<img src='" + canvas.toDataURL() +"'>");

F12打开浏览器的控制台-源代码/来源-代码段，点加号新建代码段，复制进去。然后打开视频，播放到想截图的画面，点启动就能截图，图片大小随视频分辨率变化