If this was merely a configuration (say for node.js) which sits on the server, then you probably just import a npm to read yaml (I don't write node.js so I don't know if using .yaml is feasible as node.js config file or not). So the use case is limited.

I used to work on a project which users could edit a configuration file through a web editor and we chose YAML because writing JSON by hand is painful (I hate the comma error!). But we processed this YAML file for the user on the server side, so having a native YAML parser in browser and Javascript wouldn't really help me at all.