var snapDiv = document.createElement("div");
            var html= 
                "<div class='bg'>"
                    +"<div class='top'></div>"
                    +"<div class='middle'></div>"
                    +"<div class='bottom'></div>"
             +"</div>"
             +"<div class='content'>"
                    +"<div style='margin:0 0 1px 0;'><strong>教程截图</strong></div>" 
                    +"<div style='text-align:middle;'>"
                    +"</div>"
             +"</div>" ; 
snapDiv.innerHTML = html;
snapDiv.setAttribute("id","snapDiv");
snapDiv.style.display = "none";
document.body.insertBefore(snapDiv,document.body.firstChild);