Parsing of inline HTML declaration is stricter than spec #456
Description
According to the spec (https://spec.commonmark.org/0.30/#declaration), <!A> should be a valid declaration. But cmark requires there to be a space:
$ echo 'a <!A>' | cmark <p>a <!A></p> $ echo 'a <!A >' | cmark <p>a <!-- raw HTML omitted --></p>commonmark.js has the same behavior (https://spec.commonmark.org/dingus/?text=a%20%3C!A%3E%0A%0Aa%20%3C!A%20%3E).