边框属性 | border-top

border-top

CSS属性 border-top是属性border-top-color,border-top-style, 和border-top-width 的三者的缩写。这些属性都是在描述一个元素的上方的边框border。

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

和CSS所有的缩写属性一样，border-top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-top-style: dotted;
border-top: thick green;

...实际上和...

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

并且在 border-top之前定义的 border-top-style 的值也会被覆盖。由于 border-top-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: border-top-width: medium border-top-style: none border-top-color: currentcolor
适用元素	all elements. It also applies to ::first-letter.
是否是继承属性	no
适用媒体	visual
计算值	as each of the properties of the shorthand: border-top-width: the absolute length or 0 if border-top-style is none or hidden border-top-style: as specified border-top-color: computed color
Animation type	as each of the properties of the shorthand: border-top-color: a color border-top-style: discrete border-top-width: a length
正规顺序	order of appearance in the formal grammar of the values

border-top-width：medium

border-top-style：none

border-top-color:currentcolor

Applies to all elements. It also applies to [`::first-letter`](::first-letter).   [Inherited](inheritance) no   Media visual   [Computed value](computed_value) as each of the properties of the shorthand:

border-top-width:绝对长度或0如果border-top-style是none或hidden

border-top-style:具体规定

border-top-color:计算颜色

Animation type as each of the properties of the shorthand:

border-top-color:颜色

border-top-style:离散

border-top-width:长度

Canonical order order of appearance in the formal grammar of the values

语法

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。

值

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

正式语法

<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>

示例

<div>
  This box has a border on the top side.
</div>

div {
  border-top: 4px dashed blue;
  background-color: gold;
  height: 100px;
  width: 100px;
  font-weight: bold;
  text-align: center;
}

规范

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

浏览器兼容性

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)