How do I stream a video of initially unknown size to the HTML5 video tag?

Question

I am attempting to write a web service (using C# and WebAPI, thought the actual server technology likely isn't important) that hosts dynamic video files for consumption by a basic HTML5 video element.

Here is my situation and my requirements:

• The video is being transcoded on the fly and so the final file size is not known. I cannot provide a Content-Length. However, the time duration of the video is known in advance.
• The video must be immediately playable when the page loads, even if it's not done transcoding.
• The user must be able, via the HTML5 video controls, to seek backwards and forwards in the video. If they attempt to seek forward to a point that is not transcoded, the request will block until the transcoding catches up.

I tried to use a 206: Partial Content response with the Content-Range header but since I am unable to provide a content-length, the player in Chrome seems unable to seek beyond the first chunk of video it gets and the player in Firefox doesn't even attempt to download more than the first chunk. It is also invalid to respond with a range outside what is asked for by the client but the video player always asks for bytes 0+.

Without a content-length, I considered using Transfer-Encoding: chunked and chunking the output. However, Chrome does not let you seek through a video if the server does not support ranged requests.

I have also considered specifying a fake content length of 1TB or something ridiculous just so I can do the ranged response option but I do not know if that will affect the progress bar or seek capabilities. Does the HTML5 video player determine the progress bar's dimensions based on file size or duration?

So what are my options? I am sure this is a problem that's been solved before. Can ranged responses be used in conjunction with chunked encoding?