CSS 值和单位
所有的 CSS 声明都包括一个“属性/值”对。由于属性不同,对应的值可能是单个整数或关键字,也可能是一串包含或不包含单位的关键字和值的集合。CSS 属性接受一组共同的数据类型(属性的值和对应的单位)。下面是大多数这些数据类型的概览。想要获知每一种数据类型的更详细信息,请分别查看它们的链接。
文本数据类型
<custom-ident>
- 作为
<ident>
预定义的关键字 <string>
<url>
文本数据类型要么是 <string>
,要么是一系列字符的合集,或者是 <ident>
——一个实质上是不带引号的字符串的"CSS 标识符"。一个 <string>
必须被单引号或者双引号所包围。CSS 标识符,在标准中所列出的 <ident>
或者 <custom-ident>
则必须不带引号。
在 CSS 标准中,属性的值可以由 Web 开发者指定,例如关键帧动画(keyframe animations),字体的名称(font-family names),或者是被列为 <custom-ident>
和 / 或 <string>
的栅格区域(grid-areas)。
当允许使用带引号或者不带引号的两种用户定义的文本属性值时,标准列出为 <custom-ident> | <string>
,也就是说引号是可选的。例如在指定动画的名称时:
@keyframe validIdent {
/* keyframes go here */
}
@keyframe 'validString' {
/* keyframes go here */
}
有些文本属性值不允许被引号包围。例如, grid-area
属性对应的值可以是<custom-ident>
,所以假设有一个栅格区域名为 content
,我们必须不带引号地使用:
.item {
grid-area: content;
}
相比之下,另一种数据类型是 <string>
,例如 content
属性的值则必须被引号包围:
.item::after {
content: "This is my content.";
}
大多数时候你可以随心所欲地创建任何名称作为标识符,甚至包括使用 emoji 表情。然而标识符不能是 none
,unset
,initial
,或者 inherit
,以两条短横线开头。并且,你也不可以使用任何已经预定义的 CSS 关键字。查看 <custom-ident>
和 <string>
页面了解更多信息。
预定义的关键值
预定义的关键值是由 CSS 标准为属性定义的文本值。这些关键字也属于 CSS 标识符,因此在使用时无需用引号包围。
当在 CSS 标准或者 MDN 属性页面中查看某个 CSS 属性时,该属性所有允许的取值都会以下面表格的形式列出。下面的值是 float
属性可以取的预定义值:
left | right | none | inline-start | inline-end
这些值在使用时不需要引号:
.box {
float: left;
}
CSS 全局范围内的值
URLs
<url>
类型使用一个函数符号,接收一个<string>
类型的 URL。这个 URL 可以是绝对地址或者相对地址。例如,如果你想要设置一张背景图片,那么你可以采用如下两种做法:
.box {
background-image: url("images/my-background.png");
}
.box {
background-image: url("https://www.exammple.com/images/my-background.png");
}
url()
的参数可以也可以不使用引号。如果使用引号包围了 URL,那么它会被解析为一个<url-token>
,包含对某些字符的额外转义。查看 <url>
以获取更多信息。
数值数据类型
整数
一个整数包含 0
到 9
的一个或多个十进制数字,例如 1024
或 -55
。一个整数可能额外包含 +
或 -
前缀,在正负号和数值之间没有任何空格。
数值
<number>
表示一个真正的数,有可能又或者没有小数点和小数部分。例如 0.255
,128
或 -1.2
。数值也可能包含前缀 +
或 -
标识正负。
尺寸
<dimension>
是一个包含单位的 <number>
,例如 45deg
、100ms
或 10px
。单位是大小写敏感的,且数值和单位之间不允许有任何的空格或其他字符。例如 1 cm
不是一个合法的值。
CSS 使用尺寸来指定:
<length>
(距离单位)<angle>
<time>
<frequency>
<resolution>
这些都将在下面的部分里一一介绍。
距离单位
一个距离单位,或这也称为长度(length),允许作为属性的值。它被描述为 <length>
类型。CSS 中有两种长度:相对和绝对。
相对长度单位基于其他元素的长度。例如 em
基于该元素的字号大小,vh
则与设备视口的高度有关。
单位 | 基于... |
---|---|
em |
元素的字号 |
ex |
字体的 X 字高(x-height) |
cap |
字体中大写字母的大写字高 |
ch |
Average character advance of a narrow glyph in the element’s font, as represented by the “0” (ZERO, U+0030) glyph. |
ic |
Average character advance of a full width glyph in the element’s font, as represented by the“水” (CJK water ideograph, U+6C34) glyph. |
rem |
根元素的字体大小。 |
lh |
元素的行高。 |
rlh |
根元素的行高。 |
vw |
1% of viewport's width. |
vh |
1% of viewport's height. |
vi |
1% of viewport's size in the root element's inline axis. |
vb |
1% of viewport's size in the root element's block axis. |
vmin |
1% of viewport's smaller dimension. |
vmax |
1% of viewport's larger dimension. |
Absolute length units are fixed to a physical length: either an inch or a centimeter. Many of these units are therefore more useful when the output is a fixed size media, such as print. For example, mm
is a physical millimeter, 1/10th of a centimeter.
Unit | Name | Equivalent to |
---|---|---|
cm |
Centimeters | 1cm = 96px/2.54 |
mm |
Millimeters | 1mm = 1/10th of 1cm |
Q |
Quarter-millimeters | 1Q = 1/40th of 1cm |
in |
Inches | 1in = 2.54cm = 96px |
pc |
Picas | 1pc = 1/16th of 1in |
pt |
Points | 1pt = 1/72th of 1in |
px |
Pixels | 1px = 1/96th of 1in |
When including a length value, if the length is 0
, the unit identifier is not required. Otherwise, the unit identifier is required, is case insensitive, and must come immediately after the numeric part of the value, with no space in-between.
Angle units
Angle values are represented by the type <angle>
and accept the following values:
Unit | Name | Description |
---|---|---|
deg |
Degrees | There are 360 degrees in a full circle. |
grad |
Gradians | There are 400 gradians in a full circle. |
rad |
Radians | There are 2π radians in a full circle. |
turn |
Turns | There is 1 turn in a full circle. |
Time units
Time values are represented by the type <time>
. When including a time value, the unit identifier — the s
or ms
— is required. It accepts the following values.
Unit | Name | Description |
---|---|---|
s |
Seconds | |
ms |
Milliseconds | There are 1,000 milliseconds in a second. |
Frequency units
Frequency values are represented by the type <frequency>
. It accepts the following values.
Unit | Name | Description |
---|---|---|
Hz |
Hertz | Represents the number of occurrences per second. |
kHz |
KiloHertz | A kiloHertz is 1000 Hertz. |
1Hz
, which can also be written as 1hz
or 1HZ
, is one cycle per second.
Resolution unit
Resolution units are represented by the type <resolution>
. They represent the size of a single dot in a graphical representation, such as a screen, by indicating how many of these dots fit in a CSS inch, centimeter, or pixel. It accepts the following values:
Unit | Description |
---|---|
dpi |
Dots per inch. |
dpcm |
Dots per centimetre. |
dppx , x |
Dots per px unit. |
Percentages
A <percentage>
is a type that represents a fraction of some other value.
Percentage values are always relative to another quantity, for example a length. Each property that allows percentages also defines the quantity to which the percentage refers. This quantity can be a value of another property of the same element, the value of a property of an ancestor element, a measurement of a containing block, or something else.
As an example, if you specify the width
of a box as a percentage, it refers to the percentage of the box's parent's computed width:
.box {
width: 50%;
}
Mixing percentages and dimensions
Some properties accept a dimension that could be either one of two types, for example a <length>
or a <percentage>
. In this case the allowed value is detailed in the specification as a combination unit, e.g. <length-percentage>
. Other possible combinations are as follows:
Special data types (defined in other specs)
Color
The <color>
value specifies the color of an element feature (e.g. it's background color), and is defined in the CSS Color Module.
Image
The <image>
value specifies all the different types of image that can be used in CSS, and is defined in the CSS Image Values and Replaced Content Module.
Position
The <position>
type defines 2D positioning of an object inside a positioning area, for example a background image inside a container. This type is interpreted as a background-position
and therefore specified in the CSS Backgrounds and Borders specification.
Functional notation
Functional notation is a type of value that can represent more complex types or invoke special processing by CSS. The syntax starts with the name of the function immediately followed by a left parenthesis (
followed by the argument(s) to the notation followed by a right parenthesis )
. Functions can take multiple arguments, which are formatted similarly to a CSS property value.
White space is allowed, but optional inside the parentheses. (But see notes regarding whitespace within pages for min()
, max()
and clamp()
functions.)
Some legacy functional notations such as rgba()
use commas, but generally commas are only used to separate items in a list. If a comma is used to separate arguments, white space is optional before and after the comma.
规范
Specification |
---|
CSS Values and Units Module Level 4 |
CSS Color Module Level 4 |
CSS Images Module Level 3 |