Apparently ECMAScript officially allows "HTML like" comments:
B.1.3 HTML-like Comments
The syntax and semantics of 11.4 is extended as follows except that this extension is not allowed when parsing source code using the goal symbol Module:
HTMLCloseComment:: WhiteSpaceSequence opt SingleLineDelimitedCommentSequence opt --> SingleLineCommentChars opt Which suggests that whatever comes after --> should be treated as a comment.