Developed by Microsoft and approved by Ecma International as ECMA-376 in December 2006, the Office Open XML standard, known informally as Open XML, is a document format that primarily consists of XML parts stored in a ZIP package. The Open XML standard represents word-processing documents (WordprocessingML), spreadsheets (SpreadsheetML), and presentations (PresentationML). It also includes the Open Packaging Convention standard (commonly called OPC) for storing XML, binary, and other parts in a package (most commonly as a ZIP file).
Upon approval by Ecma, the standard was then submitted to the Joint Technical Committee (JTC) 1 of International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC). Many national bodies made suggestions for improving the standard, and after Ecma made changes in response to that feedback the standard received the necessary votes and was ratified by JTC 1 and published by ISO in November 2008 as ISO/IEC 29500. Ecma has worked to keep ECMA-376 and ISO/IEC 29500 in alignment, renaming the original ECMA-376 standard as ECMA-376 1st Edition, and adopting ISO/IEC 29500 as ECMA-376 2nd Edition.
ISO/IEC 29500 standard is divided into four parts.
Open XML is the default format for Microsoft Office 2007 and Microsoft Office 2010.
Microsoft Office 2007 supports ECMA 376-1. The Open Specification site on MSDN contains detailed information about Office’s support for the ECMA-376 specification.
Microsoft Office 2010 supports ISO/IEC 29500 Transitional. Detailed information about Office’s support for ISO/IEC 29500 can be found here.
Microsoft has stated that Office will support ISO/IEC 29500 Strict Conformance no later than the next version of Office.