Change embed to isolate in example.

[r12a]

10.2.12 tts:direction
https://w3c.github.io/ttml2/index.html#style-attribute-direction

The tts:direction style is illustrated by the following example.

<p> The title of the book is "<span tts:unicodeBidi="embed" tts:direction="rtl">نشاط التدويل، W3C</span>" </p>

It is recommended to always use isolating bidi controls, rather than embedding, since it solves many problems for RTL scripts.

Therefore we recommend an editorial change to this example, to encourage good usage: change tts:unicodeBidi="embed" to tts:unicodeBidi="isolate".

[nigelmegitt]

Since this is editorial I think we could consider this change before transitioning to 2nd Ed PR. @skynavga what do you think?

[skynavga]

@nigelmegitt no, I do not agree; this requires further discussion; furthermore, the recent addition of isolate value is not reflected in current TTML/IMSC implementations;

[r12a]

It's an example. I wouldn't have thought this change would be problematic, since the spec allows one to do this. One would hope that changing the example would have the effect of encouraging people to implement support for that value, if nothing else. It's certainly desirable to isolate spans in bidrectional text.

[skynavga]

@r12a my primary concern is that the addition of isolate is a very recent change with generally unknown implementation support; the current value is not technically incorrect, so I would prefer leaving it in place for the time being, and we can revisit this issue at a future point without prejudice;

[r12a]

I still think you should be encouraging good practice rather than bad in examples.