Featurize offset times w/o metric.

[spoeschel]

Since TTML2, a <time-expression> with offset time may also be specified without a metric, in which case the seconds metric applies. The different #time-offset* features however don't consider this case in any of the syntax subsets.

[nigelmegitt]

See #113 - that issue was closed without a change. I don't think we actually agreed to make the metric optional, and I recall discussing it and agreeing that it would be a bad idea. However I can not find a documentation trail for that, so it's possible that discussion related to a different metric, e.g. on a length, and that my memory is at fault.

Regardless of that, my preference here would be to revert to the TTML1 specification and make the metric required.

[spoeschel]

I'm not aware of the history of the introduction of this change, but I would prefer to not have the ability to omit the metric as well. It is imprecise to not specify it and less easier to read for humans (possibly requires a spec lookup to be sure about that "default metric"). And of course it is not too expensive to specifiy it.

[css-meeting-bot]

The Working Group just discussed Featurize offset times w/o metric. ttml2#752, and agreed to the following:

• SUMMARY: @skynavga to prepare a pull request to remove the optionality of metric on time expressions

The full IRC log of that discussion

<nigel> Topic: Featurize offset times w/o metric. ttml2#752
<nigel> github: https://github.com//issues/752
<nigel> Glenn: I'll prepare a pull request to bring this back to TTML1 syntax by removing the
<nigel> .. optionality of metric. Then we can hold off on review until the pull request stage.
<nigel> Nigel: Sounds good.
<nigel> SUMMARY: @skynavga to prepare a pull request to remove the optionality of metric on time expressions