This property controls the thickness (width) of the top border of an element's rendering box. Negative values are not allowed.
this is a test
Explicitly sets the value of this property to that of the parent.
thin | medium | thick:
Renders a "thin", "medium" or "thick" border on the top side of the element's rendering box. The actual thickness of these border values is not specified, but "thin" should have a smaller thickness than "medium", which should have a smaller thickness than "thick".
Sets the width of the border on the top side of the element's rendering box to an explicit measurement.