Now, continuing from the comments in the original thread:
I understand that that’s a tradeoff. More confidence for the users, more work for Hugo and its maintainers.
It may not be reasonable to validate every format under the sun. Or to ship a full-blown linter. But it may be reasonable to do very basic validation for a few “blessed” formats that are easy to validate in the context of Hugo. E.g., Hugo may already have an XML parser dependency somewhere, but not a CSS parser.
If the Hugo team is absolutely against this extra maintainance burden (which would be understandable), then at least we should officially document a setup for integrating third-party validation tools. I assume that most users would want to have this validation by default.