I was reading http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35 and trying to figure out how to continue a file download.
For example, suppose a file is of length 100 bytes and I have all the 100 bytes. However, I don't know what the expected file size should be, so I ask for the file and specify a Range header that looks like this:
Range: bytes=100-
Is this a valid Range request?
For folks who are stumbling across Victor Stoddard's answer above in 2019, and become hopeful and doe eyed, note that:
a) Support for X-Content-Duration was removed in Firefox 41: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/41#HTTP
b) I think it was only supported in Firefox for .ogg audio and .ogv video, not for any other types.
c) I can't see that it was ever supported at all in Chrome, but that may just be a lack of research on my part. But its presence or absence seems to have no effect one way or another for webm or ogv videos as of today in Chrome 71.
d) I can't find anywhere where 'Content-Duration' replaced 'X-Content-Duration' for anything, I don't think 'X-Content-Duration' lived long enough for there to be a successor header name.
I think this means that, as of today if you want to serve webm or ogv containers that contain streams that don't know their duration (e.g. the output of an ffpeg pipe) to Chrome or FF, and you want them to be scrubbable in an HTML 5 video element, you are probably out of luck. Firefox 64.0 makes a half hearted attempt to make these scrubbable whether or not you serve via range requests, but it gets confused and throws up a spinning wheel until the stream is completely downloaded if you seek a few times more than it thinks is appropriate. Chrome doesn't even try, it just nopes out and won't let you scrub at all until the entire stream is finished playing.
If you're trying to make a request for content with an unknown length, and you want it to return continuous (or aggregating) response then you may want to consider use of the approach suggested in RFC8673 - namely set the last-byte-pos to 2^^53-1 so your request would look something like this:
GET /resource HTTP/1.1
Host: example.com
Range: bytes=0-9007199254740991