PHPOffice/PHPExcel — pull request #12 by alexgann (2012-07-22 19:00:16+00:00)
Title: Change BORDER_NONE mapping In HTML writer. 

Based on http://www.css3.com/css-border-collapse/ the original '1px hidden' was causing borders to mysteriously disappear.

Specifically: If any shared border has a component where the ‘border’ is set to “hidden” for ANY of the sharing members, the common border should be unconditionally set to “hidden”.