内部块级元素的高度要小于容器(父元素)
方案一：行高 = 容器高度（单行内联元素）
限制条件：仅用于单行内联元素 display:inline 和 display: inline-block;
给容器添加样式
height: 100px;line-height: 100px;
<!DOCTYPE html>
<html lang="en"><head><meta charset="UTF-8" /><meta name="viewport" content="width=device-width, initial-scale=1.0" /><title>Document</title><style>.box {border: 1px solid gray;height: 100px;line-height: 100px;}</style></head><body><div class="box"><span class="item">垂直居中 -- 内联元素 display:inline</span></div></body>
</html>
方案二：flex布局【推荐】
给容器添加样式
display: flex;
align-items: center;
<!DOCTYPE html>
<html lang="en"><head><meta charset="UTF-8" /><meta name="viewport" content="width=device-width, initial-scale=1.0" /><title>Document</title><style>.box {width: 100px;height: 100px;border: 1px solid gray;display: flex;align-items: center;}.item {width: 50px;height: 50px;border: 1px solid red;background: yellow;}</style></head><body><div class="box"><span class="item"></span></div></body>
</html>
方案三：子绝父相 + transform（CSS3）
限制条件：浏览器需支持CSS3，比较老的浏览器不适用
给容器（父元素）添加样式
position: relative给内部元素添加样式
position: absolute;
top: 50%;
transform: translate(0, -50%);<!DOCTYPE html>
<html lang="en"><head><meta charset="UTF-8" /><meta name="viewport" content="width=device-width, initial-scale=1.0" /><title>Document</title><style>.box {width: 100px;height: 100px;border: 1px solid gray;position: relative;}.item {width: 50px;height: 50px;border: 1px solid red;background: yellow;position: absolute;top: 50%;transform: translate(0, -50%);}</style></head><body><div class="box"><span class="item"></span></div></body>
</html>方案四：子绝父相 + 自动外边距（指定高度）
限制条件：内部元素需限定高度
给容器（父元素）添加样式
position: relative给内部元素添加样式
position: absolute;
top: 0;
bottom: 0;
margin: auto;<!DOCTYPE html>
<html lang="en"><head><meta charset="UTF-8" /><meta name="viewport" content="width=device-width, initial-scale=1.0" /><title>Document</title><style>.box {width: 100px;border: 1px solid gray;height: 100px;position: relative;}.item {position: absolute;top: 0;bottom: 0;margin: auto;background-color: yellow;height: 20px;}</style></head><body><div class="box"><span class="item">1111</span></div></body>
</html>
