Empty Elements Must Also Be Closed
空的元素也必须关闭

http://www.xhtml.net.cn/article_detail.asp?class=xhtml&id=152

Empty elements must either have an end tag or the start tag must end with />.
空的元素也必须有一个结束标签或者开始标签用/>结束。

This is wrong:
这是错误的：

This is a break<br>
Here comes a horizontal rule:<hr>
Here''s an image <img src="happy.gif" alt="Happy face">

This is correct:
这是正确的：

This is a break<br />

Here comes a horizontal rule:<hr />

Here''s an image <img src="happy.gif" alt="Happy face" />

To make your XHTML compatible with today''s browsers, you should add an extra space before the "/" symbol like this: <br  />, and this: <hr  />. 
为了使你的XHTML能够兼容现在的浏览器，你必须在/符号之前加一个特殊的空格,就像这样：<br  />和这样：<hr  />

原来这个空格和斜杠是这样来的啊，还一直在迷惑一些后台代码不标准呢