Add skip ink feature for text decoration.

[r12a]

This is a comment from the i18n WG, however we recognise that it comes late in the process for 2nd Edition, so we recommend that if it is not fixable before your transition, you consider it when you start work on the 3rd Edition.

10.3.37 text-decoration
https://w3c.github.io/ttml2/index.html#style-value-text-decoration

tts:textDecoration doesn't seem to support skipping ink. (See CSS for an example.) We recommend that TTML consider this for a future version.

[skynavga]

I have not yet studied this CSS3(4?) functionality, but it may already be supported, since TTML already supports removing decorations via the no{Overline,ThroughLine,Underline} values and decoration inheritance neither of which were supported in CSS2 or XSL-FO. If it does turn out that new functionality is required, then, according to the new TTML architecture rules, the addition of new substantive functionality would have to be accomplished as a TTML extension (as opposed to a feature) and accomplished in a module specification as opposed to the TTML core specification; however, in the mean time, this is being left as marked for 3ED milestone of TTML2 as a place holder.

[xfq]

Link to related CSS feature: https://drafts.csswg.org/css-text-decor-4/#text-decoration-skipping

[skynavga]

@xfq thanks for the link; I see that this functionality is not what is already supported in TTML as I mentioned above; my recommendation is that this request for new functionality be supported by concrete requirements from content authors in the subtitling/captioning communities prior to moving this request forward; IMO, this functionality could be dealt with on an implementation dependent basis, and that support for standardization is not warranted at this time, particularly given the very early status of support in CSS; in other words, if the subtitling/captioning communities aren't asking for it, then we should not be considering it here simply because CSS is looking promoting it in the more general context of i18n text on the open web platform; I would also note that if this were to be added as new functionality, then it would need to be addressed in a new extension module rather than in a core TTML specification;