A `<!-- -->` comment was written between `<` and `>`, where HTML itself does not allow one either. Inside a tag, use Zolo comments: `//` or `/* */`. In the children of an element `<!-- -->` is fine.
A full explainer for this code hasn't been written yet. The summary above is the current authoritative description.
Want to contribute? Add docs/errors/<code>.md in the repo and the page will pick it up automatically.