Question on Text-Combine-Horizontal in the Writing Mode Module

Elika,
In the description of the "all" value of "text-combine-horizontal" the following sentence occurs, "If the content contains any element boundaries this is treated as 'text-combine-horizontal: none' on the element and any descendants." What does it mean to "contain any element boundaries"?

In particular, what happens with

vertical text<span style="text-combine-horizontal: all"> H<sub>2</sub>O</span>more vertical text

I would expect that the "H2O" (with the 2 being a subscript) would be set horizontally as three characters. Is this what happens? Or, is the <sub> element considered to be an element boundary?

Steve Zilles