CSS Unit

Author: Chinaxiang Source: Internet2015-12-01 23:36

There are many length units in CSS, so I feel it is necessary to organize notes.

Overview

There are many length units in CSS, so I feel it is necessary to organize a memo.

There are also many and complete introductions online, please see the reference materials for details.

Units are roughly divided into two categories:

Absolute units will not change due to changes in the size of other elements.
Relative unit does not have a fixed measurement value, but a relative value determined by the size of other elements.

AbsolutePicas, 1in = 96px = 2.54cm

Unit	Type	Introduction
`px`	Absolute	Pixel (a point on the computer screen), `1px = 1/96in`
`pt`	Absolute	Points, `1pt = 1/72in`
##pc		1pc = 12pt
in##Absolute	Inches,
`cm`	Absolute	Centimeters, `1cm = 96/2.54px`
`mm`	Absolute	Millimeters, `1mm = 1/10cm`
`q`	Absolute	Quarter-millimeters, `1q = 1/4mm`
`%`	Relative	The width or font size relative to the parent element
`em`	Relative	Font size relative to parent element
`#rem`	Relative	(i.e. root em) The font size relative to the html tag
`ex`	Relative	The height of `x` letters in the current font environment
`ch`	Relative	The height of the number in the current font environment `0`
`vw`	Relative	1% The width of the viewport (browser visible area)
`vh`	Relative	1% The height of the viewport (visible area of the browser)
`vmin`	Relative	1% The smaller of the width and height of the viewport (browser visible area)
`vmax`	Relative	1% The larger of the width and height of the viewport (viewable area of the browser)

Since the absolute unit is a fixed value, there is nothing to introduce. The following mainly introduces the relative unit.

`%`

#The size of the same attribute relative to the parent element. This is actually not a unit, but it can be used as a unit of length after all, so it is listed here.

If used to set the font, it will be relative to the font size of the parent element.

The default font size in the <html> and <body> tags in most browsers is 100%.

html {font-size: 100%;}body {font-size: 100%;}

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100% = 1em = 16px = 12pt

If used to set non-font sizes such as width and height, the length value in percentage is based on the same attribute The length value of the parent element.

<!DOCTYPE html><html><head>
  <meta charset="utf-8">
  <title>JS Bin</title>
  <style type="text/css">
  p.parent {    margin:150px;    width: 200px;    height: 200px;    border: 1px solid blue;  }
  p.child {    width: 50%;    height: 50%;    border: 1px dashed black;  }
  </style></head><body>
  <p class="parent">
    <p class="child"></p>
  </p></body></html>

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再啰嗦一点关于父元素的问题：何为父元素，相对定位（relative），绝对定位（absolute），浮动（float），固定（fixed）中如何找寻父元素？

由于HTML是树形结构，标签套标签，一般情况下的父子关系很明朗。

<p class="parent">
    <p class="child"></p>
</p>

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相对定位元素，它的父元素符合标签嵌套。

绝对定位元素，它的父元素是离它最近的定位元素（绝对定位，相对定位，固定，但不包括浮动）或者视窗尺寸（没找到定位元素时）。

浮动元素，它的父元素也符合标签嵌套。

固定元素（特殊的绝对定位），它的父元素是视窗（浏览器默认用来展示内容的区域的尺寸，不是html 或body 的尺寸）。

注意一下绝对定位就行了，其他的相对简单。

<!DOCTYPE html><html><head>
  <meta charset="utf-8">
  <title>JS Bin</title>
  <style type="text/css">
    html {        width:1000px;    }
    body {        width:800px;    }
    #box {        width:50%;        height:300px;        position: absolute;        margin-left: 200px;        border: 1px solid red;    }
    #can {        position:absolute;        top:100px;        left:100px;        width:50%;        height:50%;        border:1px solid black;    }
  </style> </head>  <body>
    <p id="box">
        <p id="can"></p>
    </p>
    </body>  </html>

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box 宽度为视窗的一半，can 的宽高是 box 的宽高的一半。

将 can 设置为 position: fixed; 则其父元素将不再是 box 而是浏览器视窗。

can 的宽高是视窗宽高的一半。

浮动元素的父元素跟普通元素的父元素是一样的。

<!DOCTYPE html><html><head>
  <meta charset="utf-8">
  <title>JS Bin</title>
  <style type="text/css">
    html {        width:1000px;    }
    body {        width:800px;    }
    #box {        width:50%;        height:300px;        position: absolute;        margin-left: 200px;        border: 1px solid red;    }
    #can {        float:left;        width:50%;        height:50%;        border:1px solid black;    }
  </style> </head>  <body>
    <p id="box">
        <p id="can"></p>
    </p>
    </body>  </html>

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注意： padding、 margin 如果设置了百分比，上，下，左，右用的都是父元素宽度的值为标准去计算。

`em` 和 `rem`

两者都是基于字体尺寸的，区别在于 em 是相对于当前父元素的字体大小为标准，而 rem 是相对于 html 元素的字体大小为标准。

举个例子你就明白了。

<!DOCTYPE html><html><head>
  <meta charset="utf-8">
  <title>JS Bin</title>
  <style type="text/css">
  html {    font-size: 30px;  }
  body {    font-size: 14px;  }
  p.em {    font-size: 1.2em;  }
  p.rem {    font-size: 1.2rem;  }
  </style></head><body>
  <p class="em">
    Test <!-- 14 * 1.2 = 16.8px -->
    <p class="em">
      Test <!-- 16.8 * 1.2 = 20.16px -->
      <p class="em">
        Test <!-- 20.16 * 1.2 = 24.192px -->
      </p>
    </p>
  </p>
  <p  class="rem">
    Test <!-- 30 * 1.2 = 36px -->
    <p  class="rem">
      Test <!-- 30 * 1.2 = 36px -->
      <p  class="rem">
        Test <!-- 30 * 1.2 = 36px -->
      </p>
    </p>
  </p></body></html>

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`ex` and `ch`

ex and ch units, depend on the current font font- family and font size font-size. ex refers to the height of the lowercase letters x in the current font environment, and ch refers to the width of the numbers 0 in the current font environment.

Already supported by IE9+ and modern browsers.

`vw` and `vh`

Responsive web design techniques rely heavily on proportion rules. However, CSS scale is not always the best solution for every problem. What if you want to use the width or height of the display window instead of the width of the parent element? This is exactly what the vh and vw units offer.

vh is equal to the height of the viewport 1/100. For example, if the height of the browser is 900px, 1vh The obtained value is 9px. In the same way, if the display window width is 750px, the value obtained by 1vw is 7.5px.

Both units are supported by IE10+ and modern browsers.

`vmin` 和 `vmax`

这两个单位是针对vw和vh

vmin 是vw和vh中比较小的值

vmax 是vw和vh中比较大的值

.box {    height: 100vmin;    width: 100vmin;}

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.box {    height: 100vmax;    width: 100vmax;}

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IE10+ 和现代浏览器都已经支持 vmin

webkit浏览器之前不支持vmax，现在已经支持，所有现代浏览器已经支持，但是IE不支持 vmax.

总结

尺寸单位虽然说不是很难的内容，但能够做到精准理解和熟练使用也是极其难得的，也许随着CSS的发展会有更多有用的单位引进。

对单位斤斤计较是一个优秀CSS使用者应该具备的品质，赶紧去挑选合适的单位去开发你的NB产品吧。

下面的代码用来检测您的浏览器是否支持常用单位：

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
  <style type="text/css">
    body {
      padding: 20px;
    }
    div {
      background: #99ff99;
      padding: 5px;
      margin-bottom: 10px;
      white-space: nowrap;
      width: 0;
    }
    div:after {
      content: " (supported)";
    }
    div.fail {
      width: 100% !important;
      background: #ff6666 !important;
    }
    div.fail:after {
      content: " (NOT supported)";
    }
  </style> 
</head>  
<body>

<div id="percentage">50% - percentage</div>
<div id="pixel">400px - pixels (device pixels)</div>
<div id="em">20em - relative unit</div>
<div id="rem">20rem - root em</div>
<div id="vw">15vw - viewport width</div>
<div id="vh">60vh - viewport height</div>
<div id="vmin">60vmin - smaller of vw or vh</div>
<div id="vmax">60vmax - larger of vw or vh</div>
<div id="inch">4in - inches</div>
<div id="cm">20cm - centimeters</div>
<div id="mm">200mm - millimeters</div>
<div id="pt">120pt - points</div>
<div id="pc">40pc - picas</div>
<div id="ex">60ex - x-height</div>
<div id="ch">60ch - based on width of zero (0) character</div>

<script src="http://cdn.bootcss.com/jquery/1.11.3/jquery.js"></script>
<script>
// 给指定元素设置宽度
var percentage = $("#percentage").css("width", "50%");
// 如果宽度值为0,即不支持，为此元素添加fail类
if (percentage.width() == 0) percentage.addClass("fail");

var pixel = $("#pixel").css("width", "400px");
if (pixel.width() == 0) pixel.addClass("fail");

var em = $("#em").css("width", "20em");
if (em.width() == 0) em.addClass("fail");

var rem = $("#rem").css("width", "20rem");
if (rem.width() == 0) rem.addClass("fail");

var vw = $("#vw").css("width", "15vw");
if (vw.width() == 0) vw.addClass("fail");

var vh = $("#vh").css("width", "60vh");
if (vh.width() == 0) vh.addClass("fail");

var vmin = $("#vmin").css("width", "60vmin");
if (vmin.width() == 0) vmin.addClass("fail");

var vmax = $("#vmax").css("width", "60vmax");
if (vmax.width() == 0) vmax.addClass("fail");

var inch = $("#inch").css("width", "4in");
if (inch.width() == 0) inch.addClass("fail");

var cm = $("#cm").css("width", "20cm");
if (cm.width() == 0) cm.addClass("fail");

var mm = $("#mm").css("width", "200mm");
if (mm.width() == 0) mm.addClass("fail");

var pt = $("#pt").css("width", "120pt");
if (pt.width() == 0) pt.addClass("fail");

var pc = $("#pc").css("width", "40pc");
if (pc.width() == 0) pc.addClass("fail");

var ex = $("#ex").css("width", "60ex");
if (ex.width() == 0) ex.addClass("fail");

var ch = $("#ch").css("width", "60ch");
if (ch.width() == 0) ch.addClass("fail");
</script>
</body>  
</html>

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Example analysis of length unit and width adaptation in CSS

CSS Unit

Overview

%

em 和 rem

ex and ch

vw and vh

vmin 和 vmax

总结

`%`

`em` 和 `rem`

`ex` and `ch`

`vw` and `vh`

`vmin` 和 `vmax`