And then somebody says "how about we abstract this into some kind of markup language?" and we've come full circle.

A common markup language would be great! As long as we use it in a way that keeps us far away from compatibility problems.

But you have to make it extendable because you can't predict every possible use-case. So some kind of Extensible Markup Language, perhaps.

Maybe we make a sort of 'hyper' extension that comes with all kinds of baked in behaviours, so that we don't have to implement that behaviour ourselves when we deserialise it.

That's a good idea. Maybe we need a series of standards but all based on some kind of Standard Generalized markup.

You only get away from compatibility problems if either:

1. No-one uses it

2. There is only 1 implementation, and it is never updated