Each release of Ignition comes with a corresponding ignition-validate binary. There are pre-built binaries attached to each release on GitHub, as well as container images. For more details, see https://github.com/coreos/ignition#config-validation.
The fcct transpiler already validates both its input YAML and output JSON, so I don’t think you’d need further validation steps there.
I’m not aware of any online validation tools at this time. However, except for one-off manual experiments, your CI/automation pipeline should be better served by the above binaries.