watk@ is working on this.

I have an early POC: https://codereview.chromium.org/2366373003. I have quite a few open questions/API improvements/tests to write before turning it into a real CL.

Note: Mozilla is planning to drop support for this API due to developer confusion over its use.

That's interesting, do you have a link to that discussion?

Unfortunately no, it was an in-person discussion at FOMS, and I have been so far unable to follow up on it. I will keep trying.

Whoops, totally failed to post here with the responses.

Bugzilla bugs:
  - https://bugzilla.mozilla.org/show_bug.cgi?id=1022913 "fastSeek() is not guaranteed to seek in the correct direction "
  - https://bugzilla.mozilla.org/show_bug.cgi?id=1023771 "Regression: Unable to properly seek in a WEBM video"
  - https://bugzilla.mozilla.org/show_bug.cgi?id=1026330 "fastSeek should always be fast"

Some better heuristics can solve these problems except for the last one; fastSeek() is inevitably inconsistent. As a result the UX was inconsistent. "Based on that, we decided not to use fastSeek in our own built in video controls, and generally I feel fastSeek() isn't a good idea."

Their metrics (https://mzl.la/2g03mkK) showed very low adoption (but those clients that use it use it a lot).

That's interesting. If you want to chat with them, it would be interesting to know if there are any spec changes that could salvage the situation?