See the CSS2 Specs.
This paragraph has a thin double border.
This paragraph has a medium double border.
This paragraph has a thick double border.
This paragraph has a 0px double border.
This paragraph has a 1px double border.
This paragraph has a 2px double border.
This paragraph has a 3px double border.
This paragraph has a 4px double border.
This paragraph has a 5px double border.
This paragraph has a 6px double border.
This paragraph has a 7px double border.
This paragraph has a 8px double border.
This paragraph has a 9px double border.
This paragraph has a 10px double border.
This paragraph has a 11px double border.
This paragraph has a 12px double border.
This paragraph has a 13px double border.
This paragraph has a 14px double border.
This paragraph has a 15px double border.
This paragraph has a 16px double border.
This paragraph has a 17px double border.
This paragraph has a 18px double border.
This paragraph has a 19px double border.
This paragraph has a 25px double border.
This paragraph has a 50px double border.
This paragraph has a 100px double border.
(Back to CSS Testing Information, David Baron)