How can a box border width be specified?

The border-width shorthand CSS property sets the width of an element's border.

/* Keyword values */
border-width: thin;
border-width: medium;
border-width: thick;

/* <length> values */ border-width: 4px;
border-width: 1.2rem;

/* vertical | horizontal */
border-width: 2px 1.5em;

/* top | horizontal | bottom */
border-width: 1px 2em 1.5cm;

/* top | right | bottom | left */
border-width: 1px 2em 0 4rem;

/* Global keywords */
border-width: inherit;
border-width: initial;
border-width: unset;

This property is a shorthand for the following CSS properties:

border-bottom-width
border-left-width
border-right-width
border-top-width

Source: https://developer.mozilla.org/en-US/docs/Web/CSS/border-width