Clarifications on which tracks are exposed

This PR corresponds to https://www.w3.org/Bugs/Public/show_bug.cgi?id=26923
If agreeable, I'd like to make a pass on the whole spec to use normative statements (shall, must, ...) so that we can clearly check conformance on the spec.
> "A user agent may also decide to expose tracks coded in formats it does not support.."

> That makes no sense. A UA can't expose a track that it can't at least parse, which is the first step in decoding.

A user agent may also decide to expose the existence of tracks coded in formats it does not support.

Would that help?



From: Silvia Pfeiffer <notifications@github.com<mailto:notifications@github.com>>
Reply-To: w3c/HTMLSourcingInbandTracks <reply@reply.github.com<mailto:reply@reply.github.com>>
Date: Sunday, 2 November 2014 03:18
To: w3c/HTMLSourcingInbandTracks <HTMLSourcingInbandTracks@noreply.github.com<mailto:HTMLSourcingInbandTracks@noreply.github.com>>
Subject: Re: [HTMLSourcingInbandTracks] Clarifications on which tracks are exposed (#32)


In index.html:

> @@ -131,6 +131,8 @@
>        <p>
>          The specification maintains mappings from in-band audio, video and other data tracks of media resources to HTML VideoTrack, AudioTrack, and TextTrack objects and their attribute values.
>        </p>
> +      <p>This specification defines the mapping of tracks from media resources depending on the MIME type of that resource. If an implementation claims to support that MIME type and exposes a track from a resource of that type, the exposed track must conform to this specification.</p>
> +      <p>The actual tracks exposed by a user agent is implementation dependent. A user agent may expose tracks, for which it supports parsing, decoding and rendering, for playback selection by the web application or user. In that case, the track content may or may not be exposed to the web application. A user agent may also decide to expose tracks coded in formats it does not support for processing by the web application.</p>


"A user agent may also decide to expose tracks coded in formats it does not support.."
That makes no sense. A UA can't expose a track that it can't at least parse, which is the first step in decoding.

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