I started seeing docx files from the newest version of Microsoft Office at the beginning of the 2008 school year. These new MS Office Documents are called “Office Open XML”, which isn't very open and doesn't mesh well with other XML things. It is best described as a
replication of Microsoft's historical binary formats, wrapped up in angle brackets. Any new program that wants to support it would have to implement every bug, quirk, and legacy feature of all the past versions of MS Office. Why would programmers today want to keep those legacy features when none of the old versions of MS Office can read OOXML anyway? No wonder there was a disturbance in the force when Microsoft summited OOXML to become a EMCA standard, and then later a ISO standard.
During the ISO Standardization processes OOXML was separated into two variants, “Strict” and “Transitional”. Strict is the minimum set of requirements and features that are required for an OOXML document, whereas Transitional encompasses all of the legacy features that exist in documents created by MS Office 2007. In the Strict variant new documents will be internally cleaner without depending on bugs of past versions of MS office, and pave the way for other new programs to use and work with OOXML, much like an ODF from OpenOfice.
It's been two years since OOXML became a international standard, and as far as I heard the next version of MS Office won't be supporting the strict variant and will still only make transitional documents. They already broke backwards compatibility when they introduced the docx format, why shouldn't they completely make the transition and use the strict variant of OOXML? That way documents from one of the most widely used office program won't depend on just a single proprietary implementation.