This property sets the width of an element's top border. The width of the keyword values are UA dependent, but the following holds: 'thin' <= 'medium' <= 'thick'.

The keyword widths are constant throughout a document:

      H1 { border: solid thick red }
      P  { border: solid thick blue }
      

In the example above, 'H1' and 'P' elements will have the same border width regardless of font size. To achieve relative widths, the 'em' unit can be used:

      H1 { border: solid 0.5em }
      

Border widths cannot be negative.