Since it's related and the original is now offline and has only two too late captures, i'll just drop here my capture of FSFE' stance on OOXML back then

MS-OOXML: a pseudo-standard that pretends to be open

Since the very beginning of the standardisation process for Microsoft’s Office Open XML - OOXML (hereinafter MS-OOXML), the FSFE has expressed serious doubts about whether MS-OOXML could be considered as open, if even, as a standard at all.
The FSFE first raised the issue in the community and led the movement against the standardisation of MS-OOXML, following closely over the years the relevant developments.
But why can MS-OOXML still not be considered an Open Standard, despite its approval by international standards organisations?

What is Microsoft’s OOXML?

MS-OOXML is an XML-based format for office documents developed by Microsoft.
It covers word processing documents, spreadsheets, presentations, charts, diagrams, shapes, and other graphical material.
MS-OOXML was first adopted in 2006 by ECMA International - a private international standards organisation - allegedly as an Open Standard.
In 2008, the International Standards Organisation (ISO) also approved the MS-OOXML, as international Open Standard under the ISO/IEC 29500, after Microsoft requested fast-track process.
ISO approved the standard despite the fact that it was initially rejected, due to a big number of justified concerns expressed by its national member bodies.

How did we get there?

In 2001 a collective effort started to create an open document format, promoting Open Standards and trying to limit Microsoft’s office software monopoly.
The result of this effort which lasted four years, was the Open Document Format v1.0 (ODF), implemented with Free Software.
It was first approved in 2006 as an OASIS Standard and later, by ISO/IEC JTC1 as an International Standard, under the code ISO/IEC 26300:2006.
Such a format could threaten Microsoft’s dominant position in the Document market, which at the time ran on closed formats.
Under the fear of losing its power, the latter produced its own allegedly open document through a dubious ISO approval process.
As it can be imagined, the "new" format seemed in a first vote unnecessary to the ISO working group since there was already an existing document standard -ODF- and was, therefore, rejected.
However, Microsoft, in a more political than technical process, got its proposal fast-tracked even though specific clauses of it still met resistance from the national member bodies.
And this is the point in the revision process, where a suggestion came on the table, to create two sub-standards of MS-OOXML, namely ISO 29500 Transitional and ISO 29500 Strict.
Here is what happened: the Strict version got accepted by ISO, while the Transitional version was granted to Microsoft, allowing it to exclude certain legacy features and facilitating conversion from the older closed-source binary formats.
ISO then gave its approval to MS-OOXML in 2008, and justified its decision by claiming that the market needed another document standard.
It is noteworthy that Microsoft did not have, at the time of the approval, an available version of Microsoft Office that would be compliant to the ISO/IEC standard it has just passed.
That was the result of the multiple changes on the original version of the standard in order for the approval process to move forward.
Consequently, although the older binary formats (.doc, xls, and .ppt) continued to be supported by Microsoft, MS-OOXML became the default standard of all Microsoft Office documents (.docx, .xlsx, and .pptx), starting with Microsoft Office 2007.
Nevertheless, MS-OOXML Transitional is the only variant in widespread use and this is a note to keep in mind: those dual requirements led to a situation where one standard exists, and another format is fully implemented and spread all around, despite just having the status of an undocumented, proprietary specification.
In other words, something that started as an intent to enable a transitional period was turned into a norm, setting obstacles to any interoperability claims.
At a glance, Microsoft Office 2010 provides read-support for ECMA-376, read/write-support for ISO/IEC 29500 Transitional, and read support for ISO/IEC 29500 Strict.
Microsoft Office 2013 and Microsoft Office 2016 additionally support both reading and writing of ISO/IEC 29500 Strict.
However, in all of them, the default option is the transitional one, meaning that the strict standard is still not used in practice, unless the users change the default settings ad hoc.

Is MS-OOXML a true open standard?

Although many people share the assumption that some widely used formats can be identified as Open Standards, there is a number of them that does not comply with the criteria listed in the Open Standards definition.
MS-OOXML is one of them, the so-called pseudo-standards.

Dual Standards

As first objection, MS-OOXML could be seen as unnecessary.
Software engineers could instead use Open Document Format (ODF), as a less complicated office software format that was already an international standard at the time of the MS-OOXML review process, is also built upon XML technology and has the same theoretical capabilities.
Microsoft itself is a member of OASIS, the organisation in which the ODF standard was developed and where it is being maintained.
Microsoft was aware of the whole process and was invited to participate, but allegedly refused to do so, although it could have submitted its technological proposals to OASIS for inclusion into ODF.
Having two overlapping standards for the same thing creates only further burden and confusion, and can be detrimental for competition.

Compatibility and interoperability

One of the alledged main advantages of MS-OOXML was its ability to allow for backward compatibility.
However, in practice MS-OOXML seems to be compatible only with Microsoft documents, whilst its interoperability with products from other vendors appears to have been made extremely complicated by design.
This is also related to the fact that MS-OOXML was initially designed to be compatible with existing Microsoft closed formats, in comparison to ODF, which was designed with the aim to achieve document interoperability.

Supporting pre-existing Open Standards

Whenever applicable and possible, standards should build upon previous standardisation efforts and not depend on proprietary, vendor-specific technologies.
Albeit, MS-OOXML neglects various standards and uses its own vendor-specific formats instead.
This puts a substantial burden on all vendors to fully implement MS-OOXML.
It seems questionable how any third party could ever implement them equally well, especially when a standard comes with 6000 pages of specifications without serving its minimalistic purpose.

Proprietary extensions

Proprietary, application-specific extensions are a known technique employed in standardisation by vendors to abuse monopoly.
Abusive behaviour was at the core of the decision against Microsoft by the European Commission in 2004.
For this reason, it is common understanding that Open Standards should not allow such proprietary extensions, and that such market-distorting techniques should not be possible on the grounds of an Open Standard.

Application independence

Standards should not depend on certain operating systems, environments or applications, since application and implementation independence is one of the most important characteristics of all standards.
Nevertheless, MS-OOXML includes references to particular Microsoft products.

Conclusion

MS-OOXML Strict does not serve its purpose as Open Standard.
It was deliberately never implemented, to allow its transitional proprietary version to gain more ground as a default format for MS Office and to keep people locked in to proprietary solutions.
See what you can do about it.

Further reading

FSFE Context Briefing: Interoperability woes with MS-OOXML
FSFE Context Briefing: DIS-29500: Deprecated before use?
Six questions to national standardisation bodies

External links of interest

Article on Groklaw: Novell’s "Danaergeschenk"
Article on BBC: Questions for Microsoft on open formats
Article on Heise.de: The converter hoax
OOXML Abuse Index
Originalartikel

I'm reading all these comments and I guess no one here works outside of academia or 100% IT companies.

Reality out there is, that O365 is so deeply integrated with other business related software, that it's never going away. My company uses an ERP system that has maybe 200 customers worldwide. It is highly specialized for what we do. There is zero financial incentive for the manufacturer to support any other ecosystem. So they won't.

how does using docx lock users to microsoft? you can use onlyoffice to open and edit docx, ya dont need microsoft 365? pls explain to me guys

LibreOffice for life! If I ever bow to Microslop someone just end me.

Pretty noticeable that Gentoo Linux doesn't offer an option to compile OnlyOffice locally—it's only available as a -bin package, which means that it's precompiled by upstream. That tells me that either the available source is too incomplete to actually compile the software from, or it has some really strange licensing. Either way, it can't be open-source software in the accepted sense.

hmm....is it really "blasting" when it only mentions this in like a single line in the beginning paragraph?

Many interpreted the last article in this series as an attack on Microsoft for using the OOXML format against users’ interests. However, this was only one of my objectives, as I also wanted to raise users’ awareness of fake open-source software, such as OnlyOffice, which partners with Microsoft in a strategy to lock users in.

The headline doesn't match the article, but indeed onlyoffice it's not real open source.

In my language onlyoffice has several typos. Went to GitHub to submit a PR and... there are no translations to translate.

I open an issue and they tell me to send the fixes to someone by email. I send them, they are ignored and 5 years later it's still with the annoying typos.

This is a program where they pretend to be open only for marketing but they aren't actually open

You can’t make a word processor and not choose to work with docx…

What a terribly misleading post title.

The post title contains literally the entire OnlyOffice-related content of the article, which is titled "Why ODF and not OOXML". Here's the first paragraph:

Many interpreted the last article in this series as an attack on Microsoft for using the OOXML format against users’ interests. However, this was only one of my objectives, as I also wanted to raise users’ awareness of fake open-source software, such as OnlyOffice, which partners with Microsoft in a strategy to lock users in.

Everything after this is about closed vs open standards.

I've had a relatively good experience with OnlyOffice, although it has some issues.

Personally I don't see interoperability as an anti-open issue, but I can appreciate the stance. I think I have to investigate to understand how the Microsoft format diverges from the open standard for office XML files, or in what way the format remains proprietary. I had been under the impression that OnlyOffice follows the open standard.

OnlyOffice does ape Microsoft Office in a lot of ways but I see that as a positive. Users are far more likely in my opinion to switch to something that looks and feels familiar.

LibreOffice is hard to use. The menus and shortcuts are not well organized and the entire suite feels like a relic from the early 2000s. If they invested in a modern UI with less friction for users who are looking for MS alternatives, they wouldn't be facing competition from projects like OnlyOffice. If they invested in feature parity for mobile users, they wouldn't be losing potential users to those who offer it.

They have an incredibly powerful backend with far more capability than the more junior OnlyOffice. Yet they fail to recognize why that just doesn't matter to the majority of users. Most users just want to quickly author and edit files, share them with other users, and get on with the next task. LibreOffice has become overly fixated on niche features and optimizations that are very cool from a technical standpoint but are totally out of touch.

By the way, LibreOffice also supports OOXML, so... do with that what you want.

As I said in my previous comment...

BLAST

liftoff

I totally agree with LibreOffice team!