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" />
IMPORTANT Compatibility Note:
注意兼容性的关键：
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 />
