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The HTML5 specification states the following about media.seekable: "The range might be continuously changing, e.g. if the user agent is buffering a sliding window on an infinite stream. This is the behavior seen with DVRs viewing live TV, for instance." However, there is no way to implement this with MSE. It is only possible to modify the duration (the end of the seekable range), but there is no way to modify the start of the seekable range. Currently to implement DVR I have to use a custom progress bar and to translate seek requests in the proper range.
nikolay.dimitrov@viblast.com: Thank you for reporting this. We have recently moved the bug tracking for MSE to github (https://github.com/w3c/media-source/issues). Especially, please see https://github.com/w3c/media-source/issues/5 and comment on that issue whether or not the proposed solution would satisfy your concerns mentioned on this bug. In the meantime, I will close this bugzilla bug. If you deem your concerns to be distinct enough from https://github.com/w3c/media-source/issues/5, please file a new github issue for the MSE spec. With respect to maintaining a custom UI/seeking bar, that may still be required of apps for best UX, even if there is an API for modifying the seekable range(s) of an MSE-attached HTMLMediaElement. However, ability of the app to specify range(s) to which MSE seeks must be clamped in a standardized fashion was indeed deemed to be a useful MSE v.1 feature, and such specification is tracked in https://github.com/w3c/media-source/issues/5.