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The noopener keyword for the rel attribute of the <a>, <area>, and <form> elements instructs the browser to navigate to the target resource without granting the new browsing context access to the document that opened it — by not setting the Window.opener property on the opened window (it returns null).

This is especially useful when opening untrusted links, in order to ensure they cannot tamper with the originating document via the Window.opener property (see About rel=noopener for more details), while still providing the Referer HTTP header (unless noreferrer is used as well).

Source: MDN Web Docs