Why does RH lose formatting on localhost webserver, but is fine on local drive and remote webserver?

I'm using RH 2017, latest version. I have a large merged with Help system with a master project and serveral children help projects.

I have a local ISS web server (localhost) I set up to test how files look like on a real webserver locally before pushing them up to the public. However, when I view the generated help on the localhost webserver, RoboHelp files look bad, like the screen layout formatting is somehow missing?

But when I view the exact same files on the local disk drive itself, it looks fine (it also looks fine on the actual remote web server):

Any ideas why and how to resolve this? Thank you in advance.