html：

http://www.w3school.com.cn/tiy/t.asp?f=html_a_download
a标签中加上属性href: 下载链接，download: 文件名称，如果download不设置文件后缀，则默认为href中本来的格式，如果download设置了文件后缀，则弹出框中会自动变成你设置的那个后缀，下面的代码，另存为框中默认为png格式。

<a href="imgname.gif" download="givenimgname.png">
   <input type="button" value="下载">
</a>

将dom转变为canvas:

http://www.voidcn.com/blog/u013038861/article/p-6085659.html
http://blog.csdn.net/apeng_1102/article/details/50740436

<script type="text/javascript">  
    $(function(){     
        print();  
    });  
    function print(){     
       html2canvas( $("#canv") ,{            
           onrendered: function(canvas){    //这样做，DOM发生变化的不能保存
              $('#down_button').attr( 'href' , canvas.toDataURL() ) ;  
              $('#down_button').attr( 'download' , 'name.png' ) ;  
           }  
        });  
    }  
</script> 

<script  type="text/javascript" >  
    $(document).ready( function(){  
            $("#down_button").on("click", function(event) {  
                event.preventDefault();  
                html2canvas($("targrtDOM"), {  
                    allowTaint: true,  
                    taintTest: false,  
                    onrendered: function(canvas) {
                        var dataUrl = canvas.toDataURL(); //生成base64图片数据，dataUrl就是下载链接
                    }  
               });  
            });   
    });  
</script>

将canvas保存为图片：

http://www.uedsc.com/javascript-download-canvas.html

//通过toDataURL方法提取到canvas的base64编码数据
var type = 'png';
var imgData = document.getElementById(‘myCanvas’).toDataURL(type);

//mime-type改为image/octet-stream
var _fixType = function(type) {
    type = type.toLowerCase().replace(/jpg/i, 'jpeg');
    var r = type.match(/png|jpeg|bmp|gif/)[0];
    return 'image/' + r;
};

// 加工image data，替换mime type
imgData = imgData.replace(_fixType(type),'image/octet-stream');

//保存
var saveFile = function(data, filename){
    var save_link = document.createElementNS('http://www.w3.org/1999/xhtml', 'a');
    save_link.href = data;
    save_link.download = filename;
   
    var event = document.createEvent('MouseEvents');
    event.initMouseEvent('click', true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
    save_link.dispatchEvent(event);
};

// 下载后的文件名
var filename = 'qianzhui_' + (new Date()).getTime() + '.' + type;
// download
saveFile(imgData,filename);