** What is (0,0)? The top left corner of the physical page OR of the page taking into account the print margin?
** Should there be different color spaces then RGB? There is a <a href="http://www.w3.org/TR/css3-gcpm/#cmyk-colors">proposal for CMYK colors in CSS</a>.
** Should the browser display a progress view while the pages are drawn (this doesn't mean the actual printing output progress)?
** Should there be additional onBeforePrint/onAfterPrint callbacks on the PrintDocument object, or are the window.onBeforePrint/onAfterPrint events enough?
** This proposal assumes the print page size is passed during the PrintDocument constructor. A different way is to get the page size from the current selected printer's page size. Defining the page size once makes things easier but makes this API on the other side less powerful.
** Lot more... TBD
