The border-width property is used in CSS and certain HTML elements. This is a shorthand property which allows authors to specify the 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.

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. Examples of lengths are px, em, etc.

HTML example:

<h1 style="border-width:thick thin thick thin;">
H1 content.
</h1>

CSS example:

h1 {
    border-width:thick thin thick thin;
}