Basically <!> was valid, and -- only served as the start and end of a comment, and between <! and > you could have multiple "comments", example from the rfc:
<!-- another -- -- comment -->
In the current spec it is basically defined as starts with "<!--" and closed by "-->".
https://www.rfc-editor.org/rfc/rfc1866#section-3.2.5
Basically <!> was valid, and -- only served as the start and end of a comment, and between <! and > you could have multiple "comments", example from the rfc:
In the current spec it is basically defined as starts with "<!--" and closed by "-->".https://html.spec.whatwg.org/#comments