浅谈Margin和Padding值设置成百分数的布局_html/css_WEB-ITnose
转自：问说网http://www.uedsc.com/discussion-margin-and-padding-values.html
Margin和Padding是我们在网页设计经常使用到的CSS样式，他们分别是间距和填充，一个作用于盒子外面，一个作用于盒子里面，默认的情况下，这些属性的值都会被计算在盒子的面积里面，在网页开发中的流体布局或者是响应式布局中，经常将Margin和Padding设置成百分数，那么到底这个百分数是多少，他是如何计算的呢？
Margin
假设我们有这样的一段HTML代码，外面一个DIV宽度980px，高度500px，里面有一个子元素DIV，宽度和高度都不设置，然后给他设置margin:10% 5%，这个属性相当于margin:10% 5% 10% 5%。
HTML代码
<div class="demo1"> <div>这个div设置：margin:10% 5%</div></div>
CSS代码
.demo1{ height:500px; width:980px; margin:0 auto; background:#EEE; overflow:hidden;}.demo1 div{margin:10% 5%; background:#666;}为了方便查看效果，我们还分别为他们设置了不同的背景。
这里还出现了一个小的hack，就是demo1盒子不会紧挨着body，也就是不会定格布局，而且body元素上面还有一段空白，这段空白的高度刚好是.demo1 div元素的margin-top，demo1和.demo1 div元素都是顶格对其的，只要给demo1元素设置overflow:hidden，即可解决这个问题，点击这里查看示例。
我们根据以往的理解，.demo1 div的margin应该是：50px 49px 50px 49px，但是运行以后，通过查看盒模型示意图，却发现是：98px 49px 98px 49px，这是怎么回事呢？
这个98px是如何得到的呢，其实就是宽度的10%，等等，我们设置TOP为10%，不是应该参考元素的高度么，不信你可以点击这个示例页面看看效果。
总结
从上面的示例和代码，我们可以发现当margin设置成百分数的时候，其top right bottom left的值是参照父元素盒子的宽度进行计算，在w3c的规范中也是这样描述的： margin 的百分比值参照其包含块的宽度进行计算，同样的padding如果设置成百分数的话，其盒子模型和margin是一样的。
这只发生在默认的 writing-mode: horizontal-tb; 和 direction: ltr; 的情况下，当书写模式变成纵向的时候，其参照将会变成包含块的高度。
为什么要选择宽度做参照而不是高度呢？
这其实更多的要从CSS设计意图上去想，因为CSS的基础需求是排版，而通常我们所见的横排文字，其水平宽度一定（仔细回想一下，如果没有显式的定义宽度 或者强制一行显示，都会遇到边界换行，而不是水平延展），垂直方向可以无限延展。但当书写模式为纵向时，其参照就变成了高度而不再是宽度了。
延伸阅读
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