The border-width property is used to set the border width of an element by specifying between one and four values, where each value is a keyword or a length.

If four values are given, they apply to top, right, bottom, and left border width, respectively. If one value is given, it applies to all sides. If two or three values are given, the missing values are taken from the opposite side.

[ thin | medium | thick | <length> | inherit ]{1,4}

h1 { border-width: thin }                   /* thin thin thin thin */

h1 { border-width: thin thick }             /* thin thick thin thick */

h1 { border-width: thin thick medium }      /* thin thick medium thick */