the unique non-ambiguous order defined by the formal grammar
Is either a non-negative explicit <length> value or a keyword denoting the thickness of the right border. The keyword must be one of the following values:
A thin border
A medium border
A thick border
The specification doesn't define precisely the thickness of each fo the keywords, which is therefore implementation specific. Nevertheless, it requests that the thickness does follow the rule thin ≤ medium ≤ thick, and is constant on a single document.
Represents the calculated value of the border-right-width of the parent's element.