Maybe a missing dependency. It builds succesfully anyway and it doesn't talk about any problems in the console output, apart from the common useless information overload.I use Firefox and it looks fantastic to me. Nothing like what you mention happens. It must be some setting of yours.
Code:
JavaScript error: https://forums.freebsd.org/js/freebsd/freebsd-forums-mobile-layout-fixer-xf2.min.js?_v=91b29b3a, line 1: ReferenceError: jQuery is not defined