As with all shorthand properties, border-bottom 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. Consider the following code:

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

It is actually the same as this one:

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

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

This property is a shorthand for the following CSS properties:

• border-bottom-color
• border-bottom-style
• border-bottom-width

border-bottom: 1px;
border-bottom: 2px dotted;
border-bottom: medium dashed blue;

/* Global values */
border-bottom: inherit;
border-bottom: initial;
border-bottom: revert;
border-bottom: revert-layer;
border-bottom: unset;

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

<br-width>

See border-bottom-width.

<br-style>

See border-bottom-style.

<color>

See 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  

HTML

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

CSS

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

Results

• border
• border-block
• outline