Implement expanders in terms of validators. (Also fix some bugs.)
Implement expanders in terms of validators. (Also fix some bugs.)

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