[imsc] Documents that have no content may be conformant to both profiles

nigelmegitt has just created a new issue for 
https://github.com/w3c/imsc:

== Documents that have no content may be conformant to both profiles 
==
ยง5.1 General states:

> A Document Instance shall not conform to the Text Profile and Image 
Profile simultaneously.

However I cannot see any other constraints that enforce this. 
Specifically in the case of documents without content it may be 
possible to construct a document that does conform to both profiles 
simultaneously. I don't see a problem with being able to construct a 
document that conforms to both profiles, as long as it is is empty of 
TTML content. 

Note that none of the features is listed with a disposition of 
"required".

For example this document containing no content (omitting `<body>` in 
this case, and including `ttp:profile` elements in anticipation of 
#129) would be conformant to both text and image profiles 
simultaneously, and may be found for example in an ISOBMFF sample 
during which no captions or subtitles are to be displayed:

```xml
<tt
  xml:lang=""
  xmlns="http://www.w3.org/ns/ttml"
  xmlns:ttp="http://www.w3.org/ns/ttml#parameter" >
   <head>
      <ttp:profile use="http://www.w3.org/ns/ttml/profile/imsc1/image"
 />
      <ttp:profile use="http://www.w3.org/ns/ttml/profile/imsc1/text" 
/>
   </head>
</tt>
```

Even more simply, and avoiding the restriction on being conformant to 
both profiles simultaneously, the `ttp:profile` elements can be 
omitted:

```xml
<tt xml:lang="" xmlns="http://www.w3.org/ns/ttml" />
```

This, by the way, is one of the empty documents specified in [EBU 
Tech3381](https://tech.ebu.ch/publications/tech3381) "Carriage of 
EBU-TT-D in ISOBMFF".


Please view or discuss this issue at 
https://github.com/w3c/imsc/issues/136 using your GitHub account

Received on Tuesday, 19 January 2016 09:59:21 UTC