Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.
The DOMImplementation.hasFeature() method returns a boolean flag indicating if a given feature is supported. It is deprecated and modern browsers return true in all cases.
The different implementations fairly diverged in what kind of features were reported. The latest version of the spec settled to force this method to always return true, where the functionality was accurate and in use.