Comments (1)
margin概念:
margin属性为给定元素设置所有四个(上下左右)方向的外边距属性。这是四个外边距属性设置的简写。四个外边距属性设置分别是: margin-top,margin-right,margin-bottom和margin-left。指定的外边距允许为负数。
margin的top和bottom属性对非替换内联元素无效,例如span
和 code
。
实现垂直居中
想要实现垂直方向的居中可以用绝对定位:
div {
width: 20px;
height: 20px;
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
margin: auto;
}
为什么能实现垂直居中
块状水平元素,如div元素(下同),在默认情况下(非浮动、绝对定位等),水平方向会自动填满外部的容器;如果有margin-left/margin-right,padding-left/padding-right,border-left-width/border-right-width等,实际内容区域会相应变窄。
但是,当一个绝对定位元素,其对立定位方向属性同时有具体定位数值的时候,流体特性就发生了。具有流体特性绝对定位元素的margin:auto的填充规则和普通流体元素一模一样,含有以下特性:
-
如果一侧定值,一侧auto,auto为剩余空间大小;
-
如果两侧均是auto, 则平分剩余空间
更多资料
- https://developer.mozilla.org/zh-CN/docs/Web/Guide/CSS/Visual_formatting_model
- https://www.zhangxinxu.com/wordpress/2015/02/css-deep-understand-flow-bfc-column-two-auto-layout/
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