HTML中行内元素的竖直方向的padding和margin是否真的无效_html/css

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HTML中行内元素的竖直方向的padding和margin是否真的无效_html/css_WEB-ITnose

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Jun 24, 2016 am 11:21 AM

参考资料：Inline elements and padding

今天写一个导航栏时遇到了一个问题：行内元素的padding-top，padding-bottom和margin-top,margin-bottom是不是真的是无效的？

接下来就这个问题将具体分析：

1.行内元素拥有盒子模型么

答案是是的。没错，行内元素跟块级元素一样，同样拥有盒子模型。

2.行内元素的padding-top，padding-bottom和margin-top,margin-bottom是否无效

答案同样是是的。盒子模型的width，height和padding-top，padding-bottom和margin-top,margin-bottom设置都是无效的。但是...

3.实战探讨行内元素的padding-top，padding-bottom和margin-top,margin-bottom

先来看两个实例：

example1：

源码：

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>example1</title><style type="text/css">*{  margin:0px;  padding:0px;  text-decoration:none;  list-style:none;}#main{  min-width:1280px;  background-color:pink;  margin:auto;  height:800px;}nav{  height:50px;  background-color: #ffffff;}nav ul li{  height:50px;  float:left;  background-color: #ffffff;}a{  line-height:50px;  padding:30px;  margin:30px;  background-color:green;  font-size:14px;  color:rgb(231,79,77);}</style></head><body><div id="main"><header>    <nav>        <ul>            <li><a class="hnav" href="#">首页</a></li>            <li><a class="hnav" href="#">最新活动</a></li>            <li><a class="hnav" href="#">项目介绍</a></li>            <li><a class="hnav" href="#">爱心社区</a></li>            <li><a class="hnav" href="#">关于我们</a></li>        </ul>    </nav></header></div></body></html>

效果（不会做点链接弹出demo的效果，会的大神求教）：

看效果图：链接元素a的父元素li以及nav元素的高度都是50px，且都为白色背景，但是设置a的css样式为padding:30px之后，发现高度超过了白色区域（即50px），按照行内元素的盒子模型理论，应该是padding-top和padding-bottom都是无效的，然而这里却有效了么？先来看另外一个例子：

example2：

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>example2</title><style type="text/css">*{  margin:0px;  padding:0px;}span{  color:black;  padding:50px;  margin:50px;  border:3px solid black;}.pone{  color:blue;}.ptwo{  color:tomato;}</style></head><body>    <p class="pone">    test 1<span>test span</span>test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1 test 1     </p>    <p class="ptwo">test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2 test 2</p></body></html>

效果：

又是个奇怪的现象，我在截取另一张图，其他都一样，只是test 1部分的数量大量增加：

是的效果如上图，我们可以看到span设置的margin确实符合行内元素的盒子模型，水平方向外边距有效，竖直方向外边距无效，但是对于padding似乎是水平和垂直方向都有效，但我们仔细看上述example1和example2的两张图：example1中，我们设置的padding-top和padding-bottom是相等的，那么如果竖直方向的内边距真的有效，那么content的字体就应该居中，事实上并不是；example2中，我们无视边框的存在，可以看到竖直方向的内边距看不到任何效果，只有水平方向外边距有效。因此，行内元素竖直方向内边距确实是无效的

查w3c的官方文档并没有找到这个奇葩现象解释（可能我英语比较烂，没找到），后来在一篇老外的文章里找到了答案：

While padding can be applied to all sides of an inline element, only left and right padding will have an effect on surrounding content. In the example below, 50px of padding has been applied to all sides of the element. As you can see, it has an affect on the content on each side, but not on content above or below

这段话基本解释了上述现象，当我们使用内边距时，只有左右方向有效；当我们设置四个方向的内边距时，对于该行内元素，确实显示出效果，但是竖直方向的内边距只有效果，对其他元素无任何影响。

因此，行内元素的padding-top，padding-bottom和margin-top,margin-bottom是真的是无效；不过要注意一点，对于竖直方向的内边距该行内元素的内容范围是增大了，不过只是表象，对周围元素无任何影响。

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