The block HTML algorithm here allows <!-->, <!--->, etc, as comments.
These comments are also fine by the HTML parser (13.2.5.44, case for U+002D).
(note there are a couple of cases such as <!> and <!-> which HTML also allows but sees as parse errors, I am not talking about these).

The “inline” algorithm here does not allow <!--> or <!--->. They look a lot like comments, so I don’t really expect people to depend on these characters to be text. And it’s inconsistent with blocks. Can we change the spec to allow them?

I can do the work