[ttml2] Which style attributes are permitted in animate and set (#1153)

nigelmegitt has just created a new issue for https://github.com/w3c/ttml2:

== Which style attributes are permitted in animate and set ==
The `animate` and `set` elements' content model permits and style attribute in the TT Style Namespace. However:

1. The introduction to the sections on `set` and `animate` has been changed to refer to "styling attributes", but the content model does not reflect that. Although this is syntactically correct (since attributes in other namespaces are excluded from the content model and pruned prior to validation) we can do something more useful: I propose adding a comment to the content model highlighting that styling attributes in other namespaces are also considered for processing in the animation semantic, even though they are excluded from the content model from a formal specification perspective.

2. There is no constraint in `animate` that the specified styling attributes have an "Animatable" specification that includes "continuous" (not sure if "discrete" should be permitted too - I guess there's a fallback to discrete animation even if `animate` is used, so probably it should be).

3. There is no constraint in `set` that the specified styling attributes have an "Animatable" specification that includes "discrete".

The key point about 2 and 3 is that attributes that say "Animatable: none" are currently permitted in `animate` or `set`, but there is no meaningful semantic, and `<animation-value-list>` is required to have at least two entries; there is no defined fallback for when the requested animation cannot be performed.

#950 highlighted to me that this could be an issue, for example `tts:ruby` has Animatable: none.

In terms of processors, I would expect that:

a. a validating processor should reject (or warn?) when a non-animatable styling attribute is present in an `animate` or `set` element;
b. a presentation processor must ignore non-animatable styling attributes in an `animate` or `set` element.

I couldn't find any spec text to deal with either of those scenarios in the TTML2 ED.

Please view or discuss this issue at https://github.com/w3c/ttml2/issues/1153 using your GitHub account

Received on Wednesday, 4 September 2019 11:26:43 UTC