body {
    margin: 0px;
    padding: 0px;
    font-family: Arial;
    font-size: 14px;
    background-color: #AC4553;
    color: #fff;
}

#game-canvas {
    border: 5px solid #fff;

    width: 840px;
    height: 420px;
    background-color: #000;

    position:absolute; /*it can be fixed too*/
    left:0; right:0;
    top:0; bottom:0;
    margin:auto;

    /*this to solve "the content will not be cut when the window is smaller than the content": */
    max-width:100%;
    max-height:100%;
    overflow:auto;
}