http://blogs.msdn.com/syedi/archive/2009/06/20/cannot-convert-type-microsoft-sharepoint-webcontrols-cssregistration-to-system-web-ui-iattributeaccessor-beware-of-this-in-custom-master-page-for-custom-publishing-site-definition.aspxI went through an interesting case last week. One of my customer is trying to create a Custom publishing Definition and wants to integrate the definition with his custom Master page. He has created the custom master page by copying the blueband.masteren-USCommunityServer 2.1 SP1 (Build: 61025.2)