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CSS属性 border-left 是属性border-left-color, border-left-style, 和border-left-width的三者的缩写。这些属性都是在描述一个元素的左边的边框border

border-left: 1px;
border-left: 2px dotted;
border-left: medium dashed green;

这三种属性值的缩写需要按照顺序定义数值,也可以省略其中的一个或者两个。

和CSS所有的缩写属性一样,border-left 总是会设置该缩写属性所包含的全部属性值,即使开发者并没有一一指定这些值。CSS缩写属性会给没有被定义的属性一个默认的属性值。那就意味着下面这个例子......

border-left-style: dotted;
border-left: thick green;

......实际上应该是下面这个样子......

border-left-style: dotted;
border-left: none thick green;

......并且在 border-left 之前定义的 border-left-style 的值也会被覆盖。由于 border-left-style 的默认值是 none, border-style的最终结果就是没有边框。

初始值as each of the properties of the shorthand:
适用元素all elements. It also applies to ::first-letter.
是否是继承属性
适用媒体visual
计算值as each of the properties of the shorthand:
Animation typeas each of the properties of the shorthand:
正规顺序order of appearance in the formal grammar of the values

Syntax

这三种属性值的缩写需要按照顺序定义数值,也可以省略其中的一个或者两个。

Values

<br-width> 
请参阅border-left-width.
<br-style> 
请参阅border-left-style.
<color>
请参阅 border-left-color.

Formal syntax

<br-width> || <br-style> || <color>

where
<br-width> = <length> | thin | medium | thick
<br-style> = none | hidden | dotted | dashed | solid | double | groove | ridge | inset | outset
<color> = <rgb()> | <rgba()> | <hsl()> | <hsla()> | <hex-color> | <named-color> | currentcolor | <deprecated-system-color>

where
<rgb()> = rgb( [ [ <percentage>{3} | <number>{3} ] [ / <alpha-value> ]? ] | [ [ <percentage>#{3} | <number>#{3} ] , <alpha-value>? ] )
<rgba()> = rgba( [ [ <percentage>{3} | <number>{3} ] [ / <alpha-value> ]? ] | [ [ <percentage>#{3} | <number>#{3} ] , <alpha-value>? ] )
<hsl()> = hsl( [ <hue> <percentage> <percentage> [ / <alpha-value> ]? ] | [ <hue>, <percentage>, <percentage>, <alpha-value>? ] )
<hsla()> = hsla( [ <hue> <percentage> <percentage> [ / <alpha-value> ]? ] | [ <hue>, <percentage>, <percentage>, <alpha-value>? ] )

where
<alpha-value> = <number> | <percentage>
<hue> = <number> | <angle>

Example

<div>
  This box has a border on the left side.
</div>
div {
  border-left: 4px dashed blue;
  background-color: gold;
  height: 100px;
  width: 100px;
  font-weight: bold;
  text-align: center;
}

Specifications

Specification Status Comment
CSS Backgrounds and Borders Module Level 3
border-left
Candidate Recommendation No direct changes, though the modification of values for the border-left-color do apply to it.
CSS Level 2 (Revision 1)
border-left
Recommendation No significant changes.
CSS Level 1
border-left
Recommendation Initial definition.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 1.0 (Yes) 1.0 (1.7 or earlier) 4 3.5 1.0 (85)
Feature Android Edge Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support 1.0 (Yes) 1.0 (1.0) (Yes) (Yes) (Yes)

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