css margin-top属性不起作用的几个原因
原因一:
外边距合并margin-top属性失效。代码实例如下:
<code class="html">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="http://www.softwhy.com/">
<title>蚂蚁部落</title>
<style type="text/css">
.first{
width:100px;
height:100px;
background-color:red;
margin-bottom:60px;
}
.second{
width:100px;
height:100px;
background-color:green;
margin-top:40px;
}
</style>
<div class="first"></div>
<div class="second"></div>
</code>从以上代码的运行可以看出，第二个div设置的margin-top并没有生效，起作用的是第一个div的设置的margin-bottom，这里有个规律，那就是合并后的外边距的高度等于外边距的高度中的较大的一个，所以遇到此种情况可以格外注意外边距大小的设置。
原因二:
子元素和父元素也可能会导致设置的子元素上外边距失效情况，代码实例如下:
<code class="html">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="http://www.softwhy.com/">
<title>蚂蚁部落</title>
<style type="text/css">
.father{
width:300px;
height:300px;
background-color:red;
margin-top:20px;
}
.children{
width:100px;
height:100px;
background-color:blue;
margin-top:10px;
}
</style>
<div class="father">
<div class="children"></div>
</div>
</code>解决方法:
为父对象在相应的外边距方向上有边框(border)和内边距(padding)，或者为overflow属性值设置为hidden即可避免。
IE6浏览器下不会出现外边距合并现象。
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