cssutils returns None on invalid stylsheets, filter those out.
cssutils returns None on invalid stylsheets, filter those out.

prepare/TestWeasyPrint,Kozea/WeasyPrint,prepare/TestWeasyPrint,andrewleech/WeasyPrint,jasco/WeasyPrint,jasco/WeasyPrint,andrewleech/WeasyPrint,marclaporte/WeasyPrint,Kozea/WeasyPrint,marclaporte/WeasyPrint