border-bottom

border-bottom 简写属性把下边框的所有属性：border-bottom-color，border-bottom-style 与 border-bottom-width 设置到了一个声明中。这些属性描述了元素的下边框样式。

与所有的简记属性相同， border-bottom 定义了它所有可以定义的属性，即使他们并没有被显式地定义。它把所有未被显式定义的参数都设置为了默认值，这意味着这些值将被定义为：

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

它和以下的代码是等价的：

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

顺便一提，在 border-bottom 之前定义的 border-bottom-style 的值因此被忽略了。由于 border-bottom-style 并没有默认值，未被定义的 border-style 参数的值将被设置为 none，也就是无底边框。

<br-width>

见 border-bottom-width。

<br-style>

见 border-bottom-style。

<color>

见 border-bottom-color。

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

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

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

<color> = 
  <absolute-color-base>  |
  currentcolor           |
  <system-color>         |
  <device-cmyk()>        

<absolute-color-base> = 
  <hex-color> (en-US)    |
  <named-color>  |
  transparent    |
  <rgb()>        |
  <rgba()>       |
  <hsl()>        |
  <hsla()>       |
  <hwb()>        |
  <lab()>        |
  <lch()>        |
  <oklab()>      |
  <oklch()>      |
  <color()>      

<device-cmyk()> = 
  device-cmyk( <cmyk-component>{4} [ / <alpha-value> ]? )  

<rgb()> = 
  rgb( [ <percentage> | none ]{3} [ / [ <alpha-value> | none ] ]? )  |
  rgb( [ <number> | none ]{3} [ / [ <alpha-value> | none ] ]? )  

<hsl()> = 
  hsl( [ <hue> | none ] [ <percentage> | none ] [ <percentage> | none ] [ / [ <alpha-value> | none ] ]? )  

<hwb()> = 
  hwb( [ <hue> | none ] [ <percentage> | none ] [ <percentage> | none ] [ / [ <alpha-value> | none ] ]? )  

<lab()> = 
  lab( [ <percentage> | <number> | none ] [ <percentage> | <number> | none ] [ <percentage> | <number> | none ] [ / [ <alpha-value> | none ] ]? )  

<lch()> = 
  lch( [ <percentage> | <number> | none ] [ <percentage> | <number> | none ] [ <hue> | none ] [ / [ <alpha-value> | none ] ]? )  

<oklab()> = 
  oklab( [ <percentage> | <number> | none ] [ <percentage> | <number> | none ] [ <percentage> | <number> | none ] [ / [ <alpha-value> | none ] ]? )  

<oklch()> = 
  oklch( [ <percentage> | <number> | none ] [ <percentage> | <number> | none ] [ <hue> | none ] [ / [ <alpha-value> | none ] ]? )  

<color()> = 
  color( <colorspace-params> [ / [ <alpha-value> | none ] ]? )  

<cmyk-component> = 
  <number>      |
  <percentage>  

<alpha-value> = 
  <number>      |
  <percentage>  

<hue> = 
  <number>  |
  <angle>   |
  none      

<colorspace-params> = 
  <predefined-rgb-params>  |
  <xyz-params>             

<predefined-rgb-params> = 
  <predefined-rgb> [ <number> | <percentage> | none ]{3}  

<xyz-params> = 
  <xyz-space> [ <number> | none ]{3}  

<predefined-rgb> = 
  srgb          |
  srgb-linear   |
  display-p3    |
  a98-rgb       |
  prophoto-rgb  |
  rec2020       

<xyz-space> = 
  xyz      |
  xyz-d50  |
  xyz-d65