border-top-width - CSS: Cascading Style Sheets | MDN
developer.mozilla.org › Web › CSSDefines 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. medium. thick. 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 ...