border-top - CSS: Cascading Style Sheets 编辑

The border-top shorthand CSS property sets all the properties of an element's top border.

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As with all shorthand properties, border-top always sets the values of all of the properties that it can set, even if they are not specified. It sets those that are not specified to their default values. This means that ...

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

... is actually the same as ...

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

... and the value of border-top-style given before border-top is ignored. Since the default value of border-top-style is none, not specifying the border-style part results in no border.

Constituent properties

This property is a shorthand for the following CSS properties:

• border-top-color
• border-top-style
• border-top-width

Syntax

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

The three values of the shorthand property can be specified in any order, and one or two of them may be omitted.

Values

<br-width>

See border-top-width.

<br-style>

See border-top-style.

<color>

See border-top-color.

Formal definition

Initial value	as each of the properties of the shorthand: border-top-width: medium border-top-style: none border-top-color: currentcolor
Applies to	all elements. It also applies to ::first-letter.
Inherited	no
Computed value	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

Formal syntax

<line-width> || <line-style> || <color>
where 
<line-width> = <length> | thin | medium | thick
<line-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} [ / <alpha-value> ]? ) | rgb( <number>{3} [ / <alpha-value> ]? ) | rgb( <percentage>#{3} , <alpha-value>? ) | rgb( <number>#{3} , <alpha-value>? )
<rgba()> = rgba( <percentage>{3} [ / <alpha-value> ]? ) | rgba( <number>{3} [ / <alpha-value> ]? ) | rgba( <percentage>#{3} , <alpha-value>? ) | rgba( <number>#{3} , <alpha-value>? )
<hsl()> = hsl( <hue> <percentage> <percentage> [ / <alpha-value> ]? ) | hsl( <hue>, <percentage>, <percentage>, <alpha-value>? )
<hsla()> = hsla( <hue> <percentage> <percentage> [ / <alpha-value> ]? ) | hsla( <hue>, <percentage>, <percentage>, <alpha-value>? )
where 
<alpha-value> = <number> | <percentage>
<hue> = <number> | <angle>

Examples

Applying a top border

HTML

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

CSS

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

Results

Specifications

Specification	Status	Comment
CSS Backgrounds and Borders Module Level 3 The 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 1 The definition of 'border-top' in that specification.	Recommendation	Initial definition

Browser compatibility

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The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

See also

• border
• border-block
• outline