function preview(oper) {
if (oper < 10) {
bdhtml = window.document.body.innerHTML;//获取当前页的html代码
sprnstr = "<!--startprint" + oper + "-->";//设置打印开始区域
eprnstr = "<!--endprint" + oper + "-->";//设置打印结束区域
prnhtml = bdhtml.substring(bdhtml.indexOf(sprnstr) + 18); //从开始代码向后取html

prnhtml = prnhtml.substring(0, prnhtml.indexOf(eprnstr));//从结束代码向前取html
window.document.body.innerHTML = prnhtml;
window.print();
window.document.body.innerHTML = bdhtml;
} else {
window.print();
}

}
#outer {
display:table;
height:400px;
#position:relative;
overflow:hidden;
}
#middle {
display:table-cell;
vertical-align:middle;
#position:absolute;
#top:50%;
}
#inner {
#position:relative;
#top:-50%;
}



<div id=”outer”>
<div id=”middle”>
<div id=”inner”>
<!–这里是要垂直居中的内容–>
</div>
</div>
</div>