A YAML validator can catch syntax errors and point to a line, but a document that parses may still mean something different from what its author intended.
Test indentation, quoted values, lists, duplicate keys, and a value containing : or #. Check the parsed preview rather than only the green success message; some YAML parsers interpret unquoted values in surprising ways. If the file configures an application, validate required fields and types with that application's schema or a safe test run.
Keep the original file and review changes before formatting. Do not assume that valid YAML is safe or correct configuration.
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