body {
    margin: 0;
}

#content {
    width:95%;
    margin: auto;
    max-width:960px;
    border: solid grey 2px;
    position: absolute;
    top: 0; bottom: 0; right: 0; left: 0;
}

canvas {
    height:100%;
    width:100%;
}
body {
    margin: 0;
}

#content {
    width:95%;
    margin: auto;
    max-width:960px;
    max-height:540px;
    border: solid grey 2px;
    position: absolute;
    top: 0; bottom: 0; right: 0; left: 0;
}

canvas {
    height:100%;
    width:100%;
}
Set max-height to avoid start-up flicker
Set max-height to avoid start-up flicker
