There are several places in the Flexbox algorithm where it seems assumed that
the writing mode of the flexbox item is the same as that of the flexbox. This
isn't a true assumption, so the algorithm needs to be fixed to define these
Also the pagination section seems to assume that the writing mode of the
flexbox is the same as that of the fragmenter. This is also not always true.
So the issue of columns vs. rows is really one of main axis parallel to
fragmenter's block axis vs. main axis perpendicular to fragmenter's block