If your server doesn't support range headers you don't support half the features of video playback. Without range requests you can't seek to parts of the video

If your server doesn't support range headers you don't support half the features of video playback. Without range requests you can't seek to parts of the video that have yet to download. You also can't support the preload attribute. Edit: Without range requests the browser can't play a video that hasn't been saved correctly. The MP4 format supports the header atom written to the end of a file, so in cases where a file is streamed to disk a header isn't written until you know the stream is complete and can fill in the details. "Web Optimized" (there's a variety of names) are MP4s with the header atom written at the beginning of the file with the media atoms following. Without the header data you can't know much about the media tracks in the file so you can't effectively set up a decoder pipeline. A browser will request a file and if it doesn't find the header atom will use a range request to grab the last few kilobytes of the file to grab the header data. It can then set up the decode pipeline and play back the file. If the server doesn't support range headers and the file isn't "Web Optimized" a browser needs to download the whole file before it can begin playing it. HLS/DASH streams store a lot of data in the manifest file and the MPEG-TS/MP4 chunk streams contain redundant track metadata to inform the decoder pipeline.