CSS BORDER-BOTTOM-WIDTH

This property controls the thickness (width) of the bottom border of an element’s rendering box. Negative values are not allowed.

Example
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strong {
  border-bottom-width: medium
}
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<span style="border-bottom-width: 10px">This is a Test</span>

Possible Values

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 bottom 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 bottom side of the element’s rendering box to an explicit measurement.