The "syntax.txt" chapter in the official Vim documentation says:
HTML comments are rather special (see an HTML reference document for the details), and the syntax coloring scheme will highlight all errors. However, if you prefer to use the wrong style (starts with <!-- and ends with --!>) you can define: :let html_wrong_comments=1
The help text describes a "wrong style". But I didn't understand: How is the style "wrong"?