I came here for this.

I was very curious for the reason and I found it. Cssbed is not hosting css files but loading upstream css and https://vanillacss.com/vanilla.css has been hacked/hijacked, so no css is being loaded but HTML full of crap.

I thought if request origin or content-type was stylesheet no href GET should be triggered (maybe src) but looks like full HTML site is being rendered behind the scenes of a simple css request.