border-top-width

The border-top-width CSS property sets the width of the top border of an element.

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

/* <length> values */
border-top-width: 10em;
border-top-width: 3vmax;
border-top-width: 6px;

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

<line-width>

Defines the width of the border, either as an explicit nonnegative <length> or a keyword. If it's a keyword, it must be one of the following values:

thin		A thin border
medium		A medium border
thick		A thick border

Note: Because the specification doesn't define the exact thickness denoted by each keyword, the precise result when using one of them is implementation-specific. Nevertheless, they always follow the pattern thin ≤ medium ≤ thick, and the values are constant within a single document.

<line-width>where <line-width> = <length> | thin | medium | thick

<div>Element 1</div>
<div>Element 2</div>

div {
  border: 1px solid red;
  margin: 1em 0;
}

div:nth-child(1) {
  border-top-width: thick;
}
div:nth-child(2) {
  border-top-width: 2em;
}

• The other border-width-related CSS properties: border-left-width, border-right-width, border-bottom-width, and border-width.
• The other border-top-related CSS properties: border, border-top, border-top-style, and border-top-color.