What this does
Parses YAML with a full YAML 1.2 parser and tells you whether it is valid. On failure you get the error message plus the line and column, with a few lines of context around the problem.
What gets caught
Bad indentation, tabs used for indentation, unclosed flow collections like [1, 2, duplicate keys, undefined aliases, and invalid block scalars. Every document in a multi-document file is checked, and the result reports how many there are and whether the top level is a mapping, a sequence, or a scalar.
What it does not do
It checks syntax, not meaning. A valid file can still be wrong for Kubernetes, GitHub Actions, or Compose; those need their own schema.
Privacy
Everything runs in your browser. Your input is never uploaded, stored, or put in the URL.