[AnthonyMouse]

When the original application is open source, its format is public and anyone who wants to create a converter to another format can either use the original code or inspect it to see how the format works.

When the format is proprietary, you get a blob of binary data that you have to reverse engineer through guesswork, made orders of magnitude more difficult if the original service isn't even available anymore.

[est]

The problem reminds me of pdf, the format is well known, there are tons of third-party readers, but they all glitch somewhat differently.

Also PDF is turing complete.

Modern rich content formats are hard. Open formats solves only small part of the problem.

I think the universal interchangable format is static HTML.

[AnthonyMouse]

PDF is a mess because it's an ancient originally-proprietary format based on a proprietary language and the Adobe "standard" didn't actually specify a lot of important things for a long time, leaving other implementations to try to reverse engineer them.

Meanwhile you look at actual standard formats like PNG or UTF-8 or, as you say, HTML, and the implementations generally do a lot better because there is a proper spec.