I wonder if it would make sense to reuse JSON schema for this. YAML can be easily converted to JSON without loosing any information (?), so in theory YAML could be validated by the same tool chain, allowing open source community to concentrate on one good schema tool chain. The schema itself could also be written in YAML and converted to JSON.
JSON Schema can be used with most YAML documents resulting in a more portable and better documented solution than Rx or Kwalify. JSON Schema is the only of the three for which I have been able to find editor support.
Full Disclosure: I authored the web site linked above to try to make the application of JSON Schema to YAML more discoverable. I also wrote an editor extension for VS Code that has since been superseded by the RedHat extension linked above.
Good idea. Googled this up because I was looking for the same.
It's possible to convert YAML to XML in a defined manner (similarly to JSON <-> XML) and validate with a standard XML validator.
Depending on your platform, there are tools or snippets for that conversion: JavaScript (NPM), Ruby, Java (Jackson), Java (TestNG) (you'll need to see the source for what parameters it wants).
If done using an API, the error positions can even be mapped back to the original YAML file.
If your project is in C++, you can also use the yaml-schema-cpp library. It allows to validate (and complete) the .yaml input files using schema files (YAML files with extension .schema) from your C++ code.