You might think that accessibility support for a document format is nothing more than the equivalent of providing the 'alt' attribute for the <img> in HTML. The truth is that this subject is far more complex than what you are probably thinking about (it was for me). You can find a
new whitepaper on Open XML and accessibility support on
Open XML Developer. It's a great overview of how well this support has been provided in Open XML. Go over and read up on it!