web开发（4）传说的盒模型之margin_html/css

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web开发（4）传说的盒模型之margin_html/css_WEB-ITnose

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Jun 21, 2016 am 08:51 AM

margin的使用
margin的叠压现象
margin的子债父偿现象

margin的使用

先来看下代码和效果：

<!DOCTYPE html><html lang="en">    <head>        <meta charset="UTF-8">        <title>Margin</title>        <style>            /*清空标签默认样式*/             body,div {                border: 0;                margin: 0;                padding: 0;             }            div {                width: 100px;                height: 100px;                background-color: red;                /*使用外边距*/                margin: 10px;            }        </style>    </head>    <body>        <div></div>    </body></html>

使用margin的效果.png

margin的使用与padding相似（margin后边也可跟1-4个值，具体作用请参考padding）

ok,不多说简单使用了，但是在使用margin的时候一定要注意如下两种情况：1. 同级间，左右相加，上下叠压；2. 父子级，对第一个子级使用margin-top会出现子债父偿现象。

margin的叠压现象

在解释什么是叠压之前，先看段代码：

    .div1 {        width: 100px;        height: 100px;        background-color: red;        margin-bottom: 50px;    }    .div2 {        width: 100px;        height: 100px;        background-color: red;        margin-top: 30px;    }

在这里.div1与.div2是同级的div，那么二者肯定是上下结构的，问题是此时二者之间的距离是多少像素？80px？还是40px？答案是50px，因为此时有叠压现象，说白了就是谁的值大就是多少。

我们来想这样的一个例子，古代的剑客在功力高深的时候会给人一种有jian气的感觉?，假设A剑客的jian气半径是50米，B剑客的jian气是30米，那么二者之间的安全距离是多少呢？答案是50米对不对！

在web中我们称这种现象为外边距的叠压现象。

margin的子债父偿现象

依旧还是先看段代码，不过这次又效果哦！

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Margin</title><style>    /*清空标签默认样式*/    body,div {        border: 0;        margin: 0;        padding: 0;    }    .father {        width: 300px;        height: 300px;        background-color: orange;    }    .son {        width: 100px;        height: 100px;        background-color: red;        margin-top:10px;    }</style></head><body><div class="father">    <div class="son"></div></div></body></html>

margin-top添加在.son选择其中，按理说效果应该是.son(红色块)距离.father(橘黄色块)的顶部10px，但是如果真的是这样，茶哥儿会单独拿出来讲么？没关系我们来看效果就知道了：

子债父偿.png

我们会发现.son相对于.father并没有间距，相反.father却距离body有了10px的上边距，我们称此时发生了子债父偿现象。通俗易懂哈，就是儿子没有能力偿还债务所以只能老子来善后了。

这种现象是有问题的，我们在开发中一定要避免这种事情的发生，我们有三种比较成熟的方案来避免这个现象：1. 为父级添加一个border；2. 位父级添加一个新的属性overflow:hidden;3. 不适用margin，在父级中使用padding-top。

注意：子债父偿现象只会发生在对第一个子级使用margin-top时发生。

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