I am all for people experimenting and making things they like, but a domain was purchased so I can only assume a serious intent to make something of this and my only reaction is that I hope this is not taken seriously and disappears.
Buying a domain is just part of the language craft, it doesn't mean this is a serious effort to supplant JSON. Mostly it just signals that the author is serious about keeping others away from the namespace, which maml is a really good one.
There are thousands of language projects, most of them completely pointless, many of them have a domain, approximately 0 of them are "serious" in the way you're thinking. It's mostly just hobbyists, but people still try to make a name for their work and buying a domain name is a way to protect that investment.
Also """ for multiline string, I only encountered that in Python. But apparently Java, Kotlin and Swift also do that now. In js, backquote already do the trick.
And the project is targeting more on configuration file, where TOML or YAML indeed are already competitors just as historically valid as JSON.
Isn’t the spec (https://maml.dev/spec/v0.1), which says “Duplicate keys are not allowed within an object.” sufficient for that?
Or am I misunderstanding what you mean by “array keys”?
It seems the spec is silent on whether key order is significant in dictionaries, though.
JSON does have significant things wrong with it, both with the syntax and with the data types.
For example, there is no proper integer type (this is the one problem that MAML corrects with the data), and it always uses Unicode (which MAML does not correct).
However, different file formats can be helpful for different purposes.
The question then becomes: what format is that?
Seems to me more like almost no one thought the idea was any good.
(Text formats do have some benefits, but binary formats have different benefits.)