以下是HTML页面的完整来源：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head></head>
<body>
one<br>
two<br />
three<br></br>
four
</body>
</html>
有人能解释为什么当我在IE8或chrome中查看页面时，“三”和“四”之间会出现额外的空行吗？
我认为标准应该使所有浏览器看起来都一样，而且据我所见，这个页面符合XHTML过渡标准
因为XHTML规范HTML兼容性准则指定br
已经有了一些不错的答案，但只是要指出HTML5实际上指定<；br></br>应该被解析为两个＜code＞<；br>解析为text/html时启动标记。
Firefox 3.x只在怪癖模式下执行此操作，但Firefox 4在标准模式下也执行此操作。
尽管有效，这是极不寻常的代码。更可能的是，开发人员错误地使用了<；br/gt；或<；/br>；，而它们的意思是<；br/>；。因此，大多数浏览器会将两者解释为lt；br/>；。
三和四之间的额外一行是因为有两个＜code＞<；br/>它们之间的标签。第一个将内容移动到下一行，第二个将内容再向下移动一行。这是意料之中的行为。
Edit Sorry, thought it was strict for the following.
Also, <br /> tags are empty tags and so, must be closed. Because of this, I don t think that <br> is technically xhtml compliant. It should be <br />.
I.E似乎将
和两者解释为“链接中断”，并为每一个添加类似的中断。
在XHTML中，它们是相同的，如果您将文档作为application/XHTML+xml提供，则浏览器之间不会有任何差异（假设浏览器支持XHTML，而IE 8及更低版本不支持XHTML）。
如果您将文档作为text/html/code>提供，它将被视为html，而不是XHTML，并且在html<；br>是禁止使用结束标记的元素。如果包含明确的结束标记，则某些浏览器将错误更正为假定</br>是<；br>启动标记。
如果您声称XHTML是text/html，则必须遵守各种附加规则。这些在兼容性准则。此特定准则适用于永远不能有内容的元素。
充当text/html是一种黑客攻击，旨在作为向XHTML本身过渡期间的短期措施。各种因素（包括缺乏微软的支持）阻止了这一转变的完成，HTML5运动已经放弃了这一想法，转而使用可选和禁止的结束标记（但添加/>；作为语法糖）。
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