align有三个属性：
1、居中：center；
2、靠左：left；
3、靠右：right。
html中是没办法实现上下居中的，如果设置上下居中，需要通过js程序来设置。
左右居中代码如下：
<!DOCTYPE html PUBLIC "-//W3C//<a href="https://www.baidu.com/s?wd=DTD&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1d9mWwWuWR4rHbzmyDYnjbY0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6KdThsqpZwYTjCEQLGCpyw9Uz4Bmy-bIi4WUvYETgN-TLwGUv3En1R4PH0LnjTv" target="_blank" class="baidu-highlight">DTD</a>XHTML 1.0 <a href="https://www.baidu.com/s?wd=Tr&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1d9mWwWuWR4rHbzmyDYnjbY0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6KdThsqpZwYTjCEQLGCpyw9Uz4Bmy-bIi4WUvYETgN-TLwGUv3En1R4PH0LnjTv" target="_blank" class="baidu-highlight">Tr</a>ansitional//EN" "http://www.w3.org/<a href="https://www.baidu.com/s?wd=TR&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1d9mWwWuWR4rHbzmyDYnjbY0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6KdThsqpZwYTjCEQLGCpyw9Uz4Bmy-bIi4WUvYETgN-TLwGUv3En1R4PH0LnjTv" target="_blank" class="baidu-highlight">TR</a>/xhtml1/<a href="https://www.baidu.com/s?wd=DTD&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1d9mWwWuWR4rHbzmyDYnjbY0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6KdThsqpZwYTjCEQLGCpyw9Uz4Bmy-bIi4WUvYETgN-TLwGUv3En1R4PH0LnjTv" target="_blank" class="baidu-highlight">DTD</a>/xhtml1-<a href="https://www.baidu.com/s?wd=tr&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1d9mWwWuWR4rHbzmyDYnjbY0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6KdThsqpZwYTjCEQLGCpyw9Uz4Bmy-bIi4WUvYETgN-TLwGUv3En1R4PH0LnjTv" target="_blank" class="baidu-highlight">tr</a>ansitional.<a href="https://www.baidu.com/s?wd=dtd&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1d9mWwWuWR4rHbzmyDYnjbY0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6KdThsqpZwYTjCEQLGCpyw9Uz4Bmy-bIi4WUvYETgN-TLwGUv3En1R4PH0LnjTv" target="_blank" class="baidu-highlight">dtd</a>">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/htmlcharset=utf-8" />
<title>无标题文档</title>
</head>
<body>
<table width="400" height="400" border="1" align="center">
<tr align="center" valign="middle">
<td>居中</td>
<td>居中</td>
<td>居中</td>
</tr>
<tr align="center" valign="middle">
<td>居中</td>
<td>居中</td>
<td>居中</td>
</tr>
</table>
</body>
</html>
<div class="text clearfix" style="text-align:center">
2.<span style="float:right">居右</span><span style="float:left">居左</span>居中
</div>
<table width="" border="0" cellpadding="0" cellspacing="1" bgcolor="#CCCCCC">
<tr>
<td bgcolor="#FFFFFF" align="left">居左</td>
<td bgcolor="#FFFFFF" align="center">居中</td>
<td bgcolor="#FFFFFF" align="right">居右</td>
</tr>
</table>
需要准备的材料分别有：电脑、浏览器、html编辑器。
1、首先，打开html编辑器，新建html文件，例如：index.html，填充问题基础代码。
2、在index.html中的<body>标签中，输入样式代码：style="border: 5px solid blueviolettext-align: center"。
3、浏览器运行index.html页面，此时<body>标签的内容被上下左右居中了。
