This property controls the thickness (width) of the top border of an element’s rendering box. Negative values are not allowed.
<span style="border-top-width: thin">this is a test</span>
inherit: 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”.
[length]: Sets the width of the border on the top side of the element’s rendering box to an explicit measurement.