> in practice simple formats have tendency to evolve into frankenstein monster with a mess of extensions and variations (see markdown for example) Compare to O

> in practice simple formats have tendency to evolve into frankenstein monster with a mess of extensions and variations (see markdown for example) Compare to OPML, which is supposed to support other things—exactly the type of things you describe—but it requires special handling in the implementation to actually do it, so you get implementations that don't, which isn't far off from the exact situation of variants that you describe. See the previous OPML importer that I linked to for an example.