Is either a non-negative explicit <length> value or a keyword denoting the thickness of the bottom border. The keyword must be one of the following values:
A thin border
A medium border
A thick border
The specification doesn't precisely define the thickness of each of the keywords, which is therefore implementation specific. Nevertheless, it requests that the thickness does follow the thin ≤ medium ≤ thick inequality and that the values are constant on a single document.
Is a keyword indicating that all four values are inherited from their parent's element calculated value.
border: ridge #ccc;
border-width: 6px; /* same as "border: ridge #ccc 6px"; 6px wide border on all 4 sides */
border: solid red;
border-width: 2px 10px; /* 2px wide top and bottom border, 10px right and left border */
border: dotted orange;
border-width: 0.3em 0; /* 0.3em wide top and bottom border, no border right and left */
border: solid lightgreen;
border-width: medium 0 1px thick; /* three different width, no right border */