Web Design in a Nutshell, 3rd Edition

The properties may use the keyword values thin, medium, and thick, in order of increasing width. The actual pixel value for each keyword is left up to the user agent, but must be consistent throughout the document. Border width can be specified in units of length as well (pixels are common). Negative length values are not permitted for borders.

Figure 19-9 shows an example of keyword and pixel-measurement border widths.

    div {border-style: solid;
        border-top-width: thin;
        border-right-width: medium;
        border-bottom-width: thick;
        border-left-width: 12px; }

Figure 19-9. The border-width property