The [construct intermediate document] process says:

"2. evaluating this sub-tree in a postorder traversal, prune elements if
they are not a Content element, if they are temporally inactive, if they
are empty, or if they aren't associated with region R according to the
[associate region] procedure;"

This means that any empty
element e.g. "
" is pruned, resulting in it having no effect. This isn't the correct behaviour because empty
elements should create line breaks and therefore have an effect.

This applies to TTML1, TTML1SE and TTML2.

(raised by Nigel Megitt on 2015-01-29)
From tracker issue https://www.w3.org/AudioVideo/TT/tracker/issues/368