Once found, the available space should be split into columns with equal widths and the same space between them. The default is UA dependent. If a percentage is given, it is relative to the element's width. Thin, medium and thick are the same widths as for the border property. If the value is a percentage, it is relative to the element's width.
Initial value is 'medium'.
If there is a second length value, it represents the height of the rule. A percentage refers to the element's height. If no value is specified, 'none' is assumed. See the 'border' property for the meaning of each keyword. A URL for an image that is tiled to cover the rule.