Clarify that tts:visibility hidden on ancestor doesn't prevent visible on descendant.

[skynavga]

The current text on tts:visibility may be read such that if tts:visibility is hidden on an element, then all descendants are hidden, even if they specify tts:visibility as visible. This reading is not intended, so a note or additional clarifying text should be added to indicate that descendants may be visible.

[cconcolato]

I couldn't find a similar issue for TTML2 and there does not seem to be a clarification yet in. Should we create a new TTML2 issue?

[cconcolato]

In fact, I found the clarification in TTML2, https://www.w3.org/TR/ttml2/#style-attribute-visibility