This is the source of the WebVTT specification.
Contributions can be made via the W3C Text Tracks Community Group.
You can file new issues from the specification itself. There are also old bugs reported in W3C BugZilla.
This spec is generated using bikeshed.
To generate a CG draft, run:
$ bikeshed spec
To generate a WD snapshot, run e.g.:
$ ./snapshot.sh WD 2016-01-01 2015-12-08
Also see https://github.com/w3c/webvtt/commit/754f13e3cf03d6036c3e4628c6920d17b412f778 for manual fixup of the generated output.