function addWaterMarker(str){ 

    var can = document.createElement('canvas'); 

    var body = document.body;
    body.appendChild(can);
    can.width=400;
    can.height=200;
    can.style.display='none'; 

    var cans = can.getContext('2d');
    cans.rotate(-20*Math.PI/180);
    cans.font = "16px Microsoft JhengHei";
    cans.fillStyle = "rgba(17, 17, 17, 0.50)";
    cans.textAlign = 'left';
    cans.textBaseline = 'Middle';
    cans.fillText(str,can.width/3,can.height/2);
    body.style.backgroundImage="url("+can.toDataURL("image/png")+")";
}

转：https://www.cnblogs.com/lamp01/p/9377567.html