Alternatively web applications like Google Docs.

Indeed List_of_software_that_supports_Office_Open_XML is linked on the linked page.

The Office_Open_XML format has been the standard since 2007, and you don't need proprietary software to work with it.

There is a lot to unpack in that assertion, it's all the harder since Groklaw.net is now offline. You don't need proprietary software to work with OOXML but then OOXML is not .docx and part of the question here is about MS Word documents which default to the proprietary .docx series.

Yes, OOXML aka ISO/IEC 29500 is one format standard, it was whipped in great haste up to compete with the actual universal format, OpenDocument Format aka ISO 26300. Both are technically open standards, but while OOXML weighs in at well over 6,000 pages it is incompletely documented and no-one not even Microsoft implements it or even can implement it. In contrast ODF is fully documented, and fully implemented in Calligra, LibreOffice, and several others. ODF is already partially implemented in MSO, but that work appears to have stalled as MS has gone back to proprietary formats like the .docx series. Also, OOXML suffers from a tremendous amount of NIH while ODF re-uses many existing standards for components.

As for the original question, converting from markdown to HTML would be one, as mentioned by nerdvana and anonymous monk. Markdown is rather close to plain text with minimal structure and it is easy to convert between markdown and HTML using Perl. However, structure is the key and one can go from more to less but one cannot automatically produce more detail from less detail.

Milti, could you please explain more about the task?