When I submit page_styles.css to W3C I get a different message to the one you get. It says that margin-top and margin-bottom do not exist in CSS3 (but do in CSS1 and CSS2). You can use the "More Options" button in the W3C validator to select CSS2 instead.
Can Sigil be configured to use a different validator? The IDPF tool is specifically for ePub 2 and 3 documents whereas the W3C tool is for CSS in general.
Last edited by Agama; 12-06-2012 at 04:31 PM.