REST Format

REST has no real rule about what kind of format is transmitted as long as the content is based on plaintext: the developer decides whether XML, Json, or some other format is returned. Many APIs also support via HTTP Header (Content-Type) multiple formats for content transfer, e.g. Json and XML.

Why XML

Although XML has a significant overhead, unlike Json, XML can be validated extremely well and easily. XML also has a much more extensive feature set, such as comments.

Why Json

Json has clear disadvantages in terms of validation, but has a very low overhead and thus reduces bandwidth and response times. However, Json continues to evolve, so perhaps comments and other features will be supported soon.

Why YAML

There is no reason to use YAML as API format.
YAML is hell.

Please don’t use YAML!