vertical rel/pos

I'd propose the following amendment to the proposal [http://wiki.oasis-open.org/office/extension_of_vertical_relation_values_for_certain_anchor_types] which are best practice in other formats:
1. We should follow the CSS rules [http://www.w3.org/TR/2007/CR-CSS21-20070719/syndata.html#parsing-errors] for invalid arguments. This means that the attribute style:vertical-rel should be treated as if it was not there if they have invalid values.
2. If the style:vertical-rel attribute is not there then the absolute positioning style:vertical-pos should be  taken into account.
3. If a relative as well as an absolute positioning is present, then the relative positioning takes precedence over the absolute positioning.
4. We should add the same mechanism for horizontal positioning.

~Florian