A simple build of any document with --formats=html results in the HTML document referencing lang.css (from default.css via @import) but there's no lang.css in the generated tree. This looks like a return of https://bugzilla.redhat.com/show_bug.cgi?id=595799 This pollutes web logs with many 404 errors.
Actually this is not the same error. We have made many changes to the web site to make it easier to support multiple, very different, styles. We do need to document how brands need to be updated for 3.0 though.
Note that I got this problem with a brand that has been initially created with publican 3.0. So maybe you also have to update the default brand created by publican create_brand?
Too late for this now