Ghostery doesn't like AWS Beanstalk
Ghostery blocks assets from crazy AWS beanstalk endpoints.
Other malware/adware/tracker blockers probably do the same.
I spent half a day pulling my hair out over this, so I’m posting an admission of my own stupidity as a public service.
If your css and JS assets aren’t loading when you’re testing a Beanstalk service, just turn off Ghostery. That’s it.