TL; DR: It uses the HTTP link header [1] in a way that only works on some browsers:

    link <data:text/css;charset=utf-8;base64,KiB7CiAgICBiYWNrZ3JvdW5kLWNvbG9yOiBjeWFuOwp9Cgpib2R5OjphZnRlciB7CiAgY29udGVudDogIkhpISBUaGlzIGlzIHNlcnZlZCBlbnRpcmVseSBpbiB0aGUgSFRUUCBoZWFkZXJzISBQcmV0dHkgY29vbCwgaHVoPyI7CiAgZm9udC1zaXplOiAyNHB4OwogIGZvbnQtZmFtaWx5OiBjdXJzaXZlOwogIHRleHQtc2hhZG93OiAycHggMnB4IDJweCBwaW5rOwp9Cg==>; rel="stylesheet"

    * {
        background-color: cyan;
    }

    body::after {
      content: "Hi! This is served entirely in the HTTP headers! Pretty cool, huh?";
      font-size: 24px;
      font-family: cursive;
      text-shadow: 2px 2px 2px pink;
    }