IMSC1 Comments - More #4

The following statement (section 6.4) is problematic if the document
specifies

<ttp:feature value='required'>#overflow</ttp:feature>

"A Document Instance
<https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml-ww-profiles/ttml-ww-profiles.html#dfn-document-instance>
 should be authored assuming strict clipping of content that falls out of
region areas, regardless of the computed value of tts:overflow for the
region."

The reason being that if a document requires support for #overflow, then
the presentation processor must support the semantics of both 'hidden' and
'visible' values. Therefore, if this same document includes:

<region tts:overflow='visible'/>

then the author can assume that content that falls outside the region but
remains within the root container region is visible and not clipped. On the
other hand, content that falls outside the region and the root container
region may (or may not) be clipped.