https://www.yamllint.com/
YAMLlint - The YAML Validator
Validate, Verify and Reformat your YAML documents, optimized for Ruby on Rails
yaml validator
https://apinotes.io/openapi-validator
Free OpenAPI 3.2, 3.1 & Swagger Validator Online (JSON/YAML) | ApiNotes
Instant online validator for OpenAPI 3.2, 3.1, 3.0, and Swagger 2.0 specifications. Paste JSON or YAML, detect syntax errors, and preview documentation...
3 2freeopenapiswaggervalidator