In /WebCore/platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp StreamingClient::handleResponseReceived(), the HTTP Accept-Ranges header is parsed like this:
priv->seekable = length > 0 && g_ascii_strcasecmp("none", response.httpHeaderField(HTTPHeaderName::AcceptRanges).utf8().data()
This means that if the string "none" shows up in the header at all, it will match -- making it error-prone if a range using that sequence of characters is ever defined.
The syntax isn't difficult to parse, it's a comma-separated list of tokens with optional whitespace (the same as several other headers):
(found by Rodger Combs on the IETF HTTP WG mailing list)
Correction: the current implementation matches only if the header value is exactly "none", which means it'd be error-prone if the server reported some range unit other than "bytes" (the one WebKit actually supports).
Ping; noticed that Firefox fixed a similar bug earlier today.