I've put together a list of videos that I recommend to new Open XML developers who need to learn about WordprocessingML markup. These videos, for the most part, are not about developer techniques or programming with Open XML. They are all about the markup. Some of these videos are recorded recently in my on-going project to record introductory screen-casts about many aspects of Open XML. Some of these screen-casts are older, but the content is still relevant, so I've included them in the list. I am not finished recording videos on WordprocessingML, but this list is now complete enough that if you assimilate all of the information in these videos, you will have a solid grounding in many aspects of WordprocessingML.
Screen-cast series: Introduction to WordprocessingML