border

border shorthand CSS 屬性可用來設定元素的邊框。它可以設定以下的值 border-width (en-US)、border-style (en-US) 以及 border-color (en-US)。

As with all shorthand properties, any omitted sub-values will be set to their initial value. Importantly, border cannot be used to specify a custom value for border-image, but instead sets it to its initial value, i.e., none.

The border shorthand is especially useful when you want all four borders to be the same. To make them different from each other, however, you can use the longhand border-width (en-US), border-style (en-US), and border-color (en-US) properties, which accept different values for each side. Alternatively, you can target one border at a time with the physical (e.g., border-top (en-US) ) and logical (e.g., border-block-start (en-US)) border properties.

Borders and outlines are very similar. However, outlines differ from borders in the following ways:

• Outlines never take up space, as they are drawn outside of an element's content.
• According to the spec, outlines don't have to be rectangular, although they usually are.

/* style */
border: solid;

/* width | style */
border: 2px dotted;

/* style | color */
border: outset #f33;

/* width | style | color */
border: medium dashed green;

/* Global values */
border: inherit;
border: initial;
border: unset;

The border property may be specified using one, two, or three of the values listed below. The order of the values does not matter.

<line-width>

Sets the thickness of the border. Defaults to medium if absent. See border-width (en-US).

<line-style>

Sets the style of the border. Defaults to none if absent. See border-style (en-US).

<color> (en-US)

Sets the color of the border. Defaults to currentcolor if absent. See border-color (en-US).

<line-width> || (en-US) <line-style> || (en-US) <color> (en-US)
where 
<line-width> = <length> (en-US) | (en-US) thin | (en-US) medium | (en-US) thick
<line-style> = none | (en-US) hidden | (en-US) dotted | (en-US) dashed | (en-US) solid | (en-US) double | (en-US) groove | (en-US) ridge | (en-US) inset | (en-US) outset
<color> = <rgb()> | (en-US) <rgba()> | (en-US) <hsl()> | (en-US) <hsla()> | (en-US) <hex-color> | (en-US) <named-color> (en-US) | (en-US) currentcolor | (en-US) <deprecated-system-color> (en-US)
where 
<rgb()> = rgb( <percentage> (en-US){ (en-US)3} (en-US) [ (en-US) / <alpha-value> ] (en-US)? (en-US) ) | (en-US) rgb( <number> (en-US){ (en-US)3} (en-US) [ (en-US) / <alpha-value> ] (en-US)? (en-US) ) | (en-US) rgb( <percentage> (en-US)# (en-US){ (en-US)3} (en-US) , <alpha-value>? (en-US) ) | (en-US) rgb( <number> (en-US)# (en-US){ (en-US)3} (en-US) , <alpha-value>? (en-US) )
<rgba()> = rgba( <percentage> (en-US){ (en-US)3} (en-US) [ (en-US) / <alpha-value> ] (en-US)? (en-US) ) | (en-US) rgba( <number> (en-US){ (en-US)3} (en-US) [ (en-US) / <alpha-value> ] (en-US)? (en-US) ) | (en-US) rgba( <percentage> (en-US)# (en-US){ (en-US)3} (en-US) , <alpha-value>? (en-US) ) | (en-US) rgba( <number> (en-US)# (en-US){ (en-US)3} (en-US) , <alpha-value>? (en-US) )
<hsl()> = hsl( <hue> <percentage> (en-US) <percentage> (en-US) [ (en-US) / <alpha-value> ] (en-US)? (en-US) ) | (en-US) hsl( <hue>, <percentage> (en-US), <percentage> (en-US), <alpha-value>? (en-US) )
<hsla()> = hsla( <hue> <percentage> (en-US) <percentage> (en-US) [ (en-US) / <alpha-value> ] (en-US)? (en-US) ) | (en-US) hsla( <hue>, <percentage> (en-US), <percentage> (en-US), <alpha-value>? (en-US) )
where 
<alpha-value> = <number> (en-US) | (en-US) <percentage> (en-US)
<hue> = <number> (en-US) | (en-US) <angle> (en-US)

<div>I have a border, an outline, AND a box shadow! Amazing, isn't it?</div>

div {
  border: 0.5rem outset pink;
  outline: 0.5rem solid khaki;
  box-shadow: 0 0 0 2rem skyblue;
  border-radius: 12px;
  font: bold 1rem sans-serif;
  margin: 2rem;
  padding: 1rem;
  outline-offset: 0.5rem;
}