Ok, let's assume the dust has settled around the allowed WS after the signature now =)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/84007ecf0f27d2074c70955d43cc1743ee39980a

commit 84007ecf0f27d2074c70955d43cc1743ee39980a
Author: fs <fs@opera.com>
Date: Fri Sep 30 13:13:02 2016

Don't allow form-feed (U+000C) as a WebVTT signature separator

Per the WebVTT parser algorithm [1], only space, tab or newline (after
normalization) are allowed to follow the "WEBVTT" signature.

[1] https://w3c.github.io/webvtt/#webvtt-parser-algorithm

BUG= 651777 

Review-Url: https://codereview.chromium.org/2382173002
Cr-Commit-Position: refs/heads/master@{#422093}

[delete] https://crrev.com/d78edc6840a9a42afc7b50e6e714b9fc21f0c6d8/third_party/WebKit/LayoutTests/media/track/opera/track/webvtt/parsing/001-expected.txt
[modify] https://crrev.com/84007ecf0f27d2074c70955d43cc1743ee39980a/third_party/WebKit/Source/core/html/track/vtt/VTTParser.cpp
[modify] https://crrev.com/84007ecf0f27d2074c70955d43cc1743ee39980a/third_party/WebKit/Source/core/html/track/vtt/VTTParser.h

No use counter, I presume? :S

No, but for the form-feed character alone I think that would've been severe overkill - form-feed has after all never been syntactically allowed (i.e it's been part of error-handling.)

Oh, so the text track yielded no subtitles anyway, but did not fire an error event and cues.length did not throw, right?
If so, code can break as a result of the throwing...

No, there was a difference (in the spec) between syntax and the parsing algorithm (the former didn't allow form-feed while the latter did because it is more lenient by design.) There's no exceptions involved, but if you had a file with "WEBVTT<U+000C>gazonk" as the signature the load will now fail (fire 'error' instead of 'load') and cues.length will be zero (not throw. There'll also still be a track and a cue list, just empty.)

Oh, so the "load" event will not fire from now on. I agree it is an edge case, but I am not completely comfortable without a use counter. But I guess I am just paranoid and the ship has sort of sailed.

Thank you.