border-top

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

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

尝试一下

与所有缩写属性(shorthand properties)一样,border-top始终设置它可以设置的所有属性的值,即使未指定它们也是如此。它将未指定的那些设置为其默认值。这意味着

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

和下面的代码效果相同:

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

border-top前指定的border-top-style将会失效。因为border-top-style 默认值为none,故没有边界即没有指定border-style

Syntax

css
border-top: 1px;
border-top: 2px dotted;
border-top: 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。

Values

形式语法

border-top = 
<line-width> ||
<line-style> ||
<color>

<line-width> =
<length [0,∞]> |
thin |
medium |
thick

<line-style> =
none |
hidden |
dotted |
dashed |
solid |
double |
groove |
ridge |
inset |
outset

示例

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;
}

规范

Specification
CSS Backgrounds and Borders Module Level 3
# border-shorthands

浏览器兼容性

BCD tables only load in the browser