According to https://www.ietf.org/rfc/rfc3986.txt (which we don't follow precisely but somewhat):

gen-delims = ":" / "/" / "?" / "#" / "]" / "[" / @"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "="
userinfo = *( unreserved / pct-encoded / sub-delims / ":" )

But with node.js, when I tried

• username of: "kay:kay" in my connection string, node.js did not let me connect even though I escaped the colon in my connection string.
• username of "k?y" also didn't work.

Linking to https://jira.mongodb.org/browse/SPEC-618 such that if the behavior across drivers are supposed to be consistent or if differences are allowed.