Illustrated Glossary of Organic Chemistry

Significant resonance contributor: A resonance contributor having less than three resonance contributor preference rule violations. A resonance contributor that violates three or more resonance contributor preference rules is not a significant resonance contributor.

Formaldehyde resonance contributor A is a significant resonance contributor because it does not violate any of the the resonance contributor preference rules. Contributor B is not a significant resonance contributor because it violates three resonance contributor preference rules: it has an avoidable open valence shell, avoidable formal charges, and less than the maximum number of covalent bonds.