Saturday, 9 March 2013

WebVTT Parser Rules

In some recent discussion the nature of the specification has come up again. Since this has never been dealt with, I was told to ignore and leave the issue, an update seems in order.

There is a new bug that has been posted where the following is stated.
"Also add a notice at syntax specification that implementers need to read the parsing section."
- Bug 21230 - [WebVTT] specify extension points in syntax spec

This comes from a discussion on the mailing list. I'll add the following relevant sections.

[Discussion of conflict between syntax and parser rules in WebVTT specification]

No, there is no conflict. The first one is the current spec, the second is the requirement on how to parse it so that the current spec can be extended in the future.

- Silvia Pfeiffer


Ralph Giles wrote:
"If you do want to do something application-specific here, at least try to follow the syntax rules implied by Silvia's draft. Then when you have some implementation experience, we should try to spec what those rules actually are."

Please don't do that, either. Don't put anything in there at all until we're sure of what the format should be.

- Glenn Maynard


Just to clarify (at least to my understanding): The first is the file format, and the second is the parser.  Both are part of the spec.  The file format tells you what's valid--what authors should be writing.  The parser tells you how to deal with every possible input, which includes error handling (inputs that don't follow the file format) and--as you said--forwards-compatibility.

I've seen a couple people confused by this, leading to people trying to write implementations by looking at the file format, which is bad.  Browser vendors understand it, since that's how the HTML spec works, but since non-browser people without experience with that spec may be implementing this (more than most other parts of the web), maybe there should be a brief note at the top of the file format section explaining this.  ("If you're implementing WebVTT, you're in the wrong place!")

- Glenn Maynard


Agreed, we need to clarify this. I want to make sure to specify extension points in the syntax specification to make sure that implementers are made aware of this.

- Silvia Pfeiffer

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