18.336 style:rotation-align has the statement (with respect to alignments "bottom", "top", "center") that
Borders and background are ... Positioned parallel to the text, whereby the upper or lower edge is drawn at the original position of the cell."

At least to me this is anything but clear. The "original position of the cell is quite large and so where exactly should the edge (which edge?) be drawn?

The rotation in question is given by the angle and this alignment option. Even the alignment option "none" is not quite clear: we know that the borders and background do not get rotated but it isn't really clear what the center of rotation for the text is.