深入理解CSS中的长度单位_html/css_WEB-ITnose
× 目录 [1]px [2]in [3]cm [4]mm [5]q [6]pt [7]pc [8]em [9]rem [10]ex [11]ch [12]vh [13]vw [14]vmin [15]vmax
前面的话
本文分为绝对长度单位和相对长度单位来介绍CSS中的长度单位的主要知识
绝对长度单位
绝对长度单位代表一个物理测量
像素px(pixels)
在web上，像素px是典型的度量单位，很多其他长度单位直接映射成像素。最终，他们被按照像素处理
英寸in(inches)
1in = 2.54cm = 96px
厘米cm(centimeters)
1cm = 10mm = 96px/2.54 = 37.8px
毫米mm(millimeters)
1mm = 0.1cm = 3.78px
1/4毫米q(quarter-millimeters)
1q = 1/4mm = 0.945px
点pt(points)
1pt = 1/72in = =0.0139in = 1/72*2.54cm = 1/72*96px = 1.33px
派卡pc(picas)
1pc = 12pt = 1/6in = 1/6*96px = 16px
字体相关相对长度单位
em、ex、ch、rem是字体相关的相对长度单位
em
em表示元素的font-size属性的计算值，如果用于font-size属性本身，相对于父元素的font-size；若用于其他属性，相对于本身元素的font-size
<style>.box{font-size: 20px;}.in{ /* 相对于父元素，所以2*2px=40px */ font-size: 2em; /* 相对于本身元素，所以5*40px=200px */ height: 5em; /* 10*40px=400px */ width: 10em; background-color: lightblue;}</style>
<div class="box"> <div class="in">测试文字</div> </div>
rem
rem是相对于根元素html的font-size属性的计算值
兼容性: IE8-不支持
<style>/* 浏览器默认字体大小为16px，则2*16=32px，所以根元素字体大小为32px */html{font-size: 2rem;}/* 2*32=64px */.box{font-size: 2rem;}.in{ /* 1*32=32px */ font-size: 1rem; /* 1*32=32px */ border-left: 1rem solid black; /* 4*32=128px */ height: 4rem; /* 6*32=192px */ width: 6rem; background-color: lightblue;}</style>
<div class="box"> <div class="in" id="test">测试文字</div> </div>
ex
ex是指所用字体中小写x的高度。但不同字体x的高度可能不同。实际上，很多浏览器取em值一半作为ex值
[注意]ex在实际中常用于微调
<style>.box{font-size: 20px;}.in{ font-size: 1ex; border-left: 1ex solid black; height: 10ex; width: 20ex; background-color: lightblue;}</style>
<div class="box"> <div class="in" id="test">测试文字</div> </div>
<script>var aBtns = document.getElementsByTagName('button');for(var i = 0; i < aBtns.length; i++ ){ aBtns[i].onclick = function(){ test.style.fontFamily = this.innerHTML; }} </script>ch
ch与ex类似，被定义为数字0的宽度。当无法确定数字0宽度时，取em值的一半作为ch值
兼容性: IE8-不支持
[注意]ch在实际中主要用于盲文排版
<style>.box{font-size: 20px;}.in{ font-size: 1ch; border-left: 1ch solid black; height: 10ch; width: 20ch; background-color: lightblue;}</style>
<div class="box"> <div class="in" id="test">测试文字</div> </div>
<script>var aBtns = document.getElementsByTagName('button');for(var i = 0; i < aBtns.length; i++ ){ aBtns[i].onclick = function(){ test.style.fontFamily = this.innerHTML; }} </script>
视口相关相对长度单位
视口相关的长度值相对于初始包含块的大小。当初始包含块的宽高变化时，他们都会相应地缩放。然而，当根元素的overflow值为auto时，任何滚动条会假定不存在。
关于视口相关的单位有vh、vw、vmin、vmax4个单位
兼容性:IE8-不支持，IOS7.1-不支持，android4.3-不支持(对于vmax，所有IE浏览器都不支持)
[注意]黑莓错误的将其相对于视觉视口来计算；而safari奇怪地相对于html元素来计算，如果html中增加了内容，这两个单位也会发生变化
vh
布局视口高度的 1/100
vw
布局视口宽度的 1/100
<style>body{margin: 0;}.box{ /* 实现与屏幕等高的效果 */ height: 100vh; background-color: lightblue;} </style>
<div class="box"></div>
vmin
布局视口高度和宽度之间的最小值的 1/100
/*类似于contain效果*/.box{ height: 100vmin; width: 100vmin;}
vmax
布局视口高度和宽度之间的最大值的 1/100
/*类似于cover效果*/.box{ height: 100vmax; width: 100vmax;}
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