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