What is an Accept-Ranges header?
Tells the client whether the server supports range requests for a resource, e.g. Accept-Ranges: bytes.
Accept-Ranges advertises whether a website's server supports partial content requests for a given resource, which is what lets a video player seek instantly or a download manager resume an interrupted transfer instead of starting over. If a site claims to support ranges but a Range request actually fails, that's a real functional bug worth catching with a monitor that tests the behaviour directly rather than just trusting the header.
Common use case
A video streaming platform sends Accept-Ranges: bytes on its video files, telling players they can seek to any point instead of only ever streaming from the beginning.
Example
Accept-Ranges: bytesHistory
Part of the original HTTP/1.1 specification, RFC 2068 (1997).
Did you know?
A server can also explicitly send Accept-Ranges: none to actively tell clients not to bother trying range requests at all, rather than just omitting the header.