The CORS error is a red herring. Stylesheets are failing to load with 404, and those 404 also happen not to have the CORS headers (since they are simple HTML er

The CORS error is a red herring. Stylesheets are failing to load with 404, and those 404 also happen not to have the CORS headers (since they are simple HTML error pages). $ http https://assets-cdn.github.com/assets/frameworks-9e1db7aa5e35573e7a01331c4afc4d3df80c21f118c6c727900d5e032e7dd7b4.css HTTP/1.1 404 Not Found Accept-Ranges: bytes Age: 3 Connection: keep-alive Content-Encoding: gzip Content-Length: 93319 Content-Type: text/html Date: Tue, 05 Sep 2017 18:10:52 GMT Server: GitHub.com Vary: Accept-Encoding Via: 1.1 varnish X-Cache: HIT X-Cache-Hits: 1 X-Fastly-Request-ID: 362b40a8dbaf695c1933af2ceba160fd011f904d X-GitHub-Request-Id: B246:7E91:7D676E:8B2E5A:59AEE8A7 X-Served-By: cache-jfk8145-JFK X-Timer: S1504635052.455916,VS0,VE2 timing-allow-origin: https://github.com EDIT: CORS is not normally required for stylesheets, but GitHub uses the "crossorigin" <link> attribute, supposedly to prevent cookies from being sent with the request, and that has the side effect of requiring CORS headers: <link crossorigin="anonymous" href="https://assets-cdn.github.com/assets/frameworks-9e1db7aa5e35573e7a01331c4afc4d3df80c21f118c6c727900d5e032e7dd7b4.css" integrity="sha256-nh23ql41Vz56ATMcSvxNPfgMIfEYxscnkA1eAy5917Q=" media="all" rel="stylesheet" /> https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link https://developer.mozilla.org/en-US/docs/Web/HTML/Element/li...