Comment by FiloSottile

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/li...