Yes, while processing the file with LaTeX, which is your use case with the Markdown package, the expl3 programming layer is surely worth considering. My point was just to say that the key-value arguments in the kind of output I propose would not hinder portability across formats, since we can at least resort to xkeyval for those that do not handle such arguments natively!

Note that you can load expl3 with \input expl3-generic\relax in both plain TeX and ConTeXt and it will just work for the most part. As the LaTeX3 Interface Documentation says:

> The fundamental programming code can also be loaded with other TeX formats, subject to restrictions on the full range of functionality.

In my opinion, this is entirely too modest of the LaTeX team to say. I have used expl3 in library code that runs with both plain TeX and ConTeXt and all works as expected with the exception of some minor issues with the l3file module in ConTeXt (now fixed).