So, it seems more like it's indicating Brotli compression, but the actual file (https://www.w3.org/2008/site/js/main) is gz encoded.

    <link rel="stylesheet" href="/2008/site/css/minimum" type="text/css" media="all" />
    <style type="text/css" media="print, screen and (min-width: 481px)">
    /*<![CDATA[*/
    @import url("/2008/site/css/advanced");
    /*]]>*/
    </style>
    <link href="/2008/site/css/minimum" rel="stylesheet" type="text/css" media="only screen and (max-width: 480px)" />
    <meta name="viewport" content="width=device-width" />
    <link rel="stylesheet" href="/2008/site/css/print" type="text/css" media="print" />

    Content-Type: text/css;charset=utf-8
    Transfer-Encoding: chunked
    Connection: keep-alive
    content-location: minimum.css.gz
    vary: negotiate,Accept-Encoding

Seems to be missing the Content-Encoding header?!