BuddyPress.org

OK, responding to my own questions, just not to create work for anyone else who ends up here in the future.

I had a bug in my NginX conf file. For all CSS files that were in subdirectories off the root, I was sending those files through PHP rather than just returning them. That caused the Content-Type to be set to “text/html”, rather than “text/css”.

That’s why it worked without a Doctype (quirks mode kicked in), and failed in Firefox and Chrome with the Doctype in.