CSS BORDER-WIDTH

This is a shorthand property which allows an author to specify ‘border-top-width’, ‘border-right-width’, ‘border-bottom-width’, and ‘border-left-width’ properties using a single property and value notation (the values are given in this order separated by spaces.) If one or more of the values are not present, the value for a missing side is taken from the opposite side that is present. If only one value is listed, it applies to all sides.

Example
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strong {
  border-width: thick thin
}
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<span style="border-width: thick thin thick thin">some text</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 for a 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 for a side of the element’s rendering box to an explicit measurement.