Hello,
I have a XML part of a DOCX which in fact is a w:hdr. It contains a w:drawing with a wp:anchor. Nested in this anchor element I have several subelements: wp:simplePo, wp:positionH, wp:positionV, wp:extent, wp:effectExtent, wp:wrapTight, wp:docPr, wp:cNvGraphicFramePr, a:graphic (so far everything is fine), wp14:sizeRelH and wp14:sizeRelV.
The "Office Drawing Extensions to Office Open XML Structure Sepcification" - I found a version dated June 8, 2011 - suggests this structure being valid. Older sources ...