Edit in JSFiddle

navigator.getUserMedia || (navigator.getUserMedia = navigator.mozGetUserMedia || navigator.webkitGetUserMedia || navigator.msGetUserMedia);

if (!navigator.getUserMedia) {
    return;
}

var btn = document.getElementById('run');
btn.onclick = startWebcam;

function startWebcam(e) {
    navigator.getUserMedia({
        video: true,
        audio: true
    }, onSuccess, onError);

    function onSuccess(stream) {

        var video = document.getElementById('webcam');

        if (window.URL) {
            video.src = window.URL.createObjectURL(stream);
        } else {
            video.src = stream;
        }

        video.autoplay = true;
        //or video.play();
    }

    function onError() {}
}

<button id="run">开启网络摄像头</button><Br/>
<video id="webcam"></video>