Strictly OOXML for Office 2013 says Microsoft

In the end, both Open XML and ODF were standardised by the ISO, but Microsoft didn’t switch to supporting “Strict” Open XML, working instead with a version of the standard called “Transitional” Open XML. This version of the standard was designed to be used on a temporary basis while the company switched over to the ISO-approved OOXML and pulled in a number of legacy elements such as VML (Vector Markup Language) to ensure compatibility. Microsoft pushed back its plans to switch, releasing Office 2007 without support for it and Office 2010 with only the ability to open and edit, not save, Strict OOXML documents.