It says to use the box-shadow css3 property, but only in WebKit, the rendering engine behind Google Chrome, Safari, (Opera, I think) and incidentally, the ebook-viewer. See:
http://www.css3.info/preview/box-shadow/
it requires/(required?) the -webkit- prefix because back in the day, this was experimental, and -webkit- or -moz- would be ignored by other rendering engines.