Border Top Width
{border-top-width: XXunits/thin/medium/thick}
The border-top-width property lets you set the width of an element's top border.

It is illegal to use negative values for border-top-width. Different Web browsers interpret thin, medium, and thick differently, but the width of thin is always smaller than medium, which is always smaller than thick.

The border-top-width property should be used in conjunction with the border-style property to ensure that CSS-enabled browsers process it correctly.

Example:

P {border-top-width: 1cm}

If you are using a CSS-enabled browser, this paragraph has a 1-centimeter-tall top border.