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The X-XSS-Protection header is not defined. This header can hint to the user agent to protect against some forms of XSS

Description

Cross-Site Scripting (XSS) attacks occur when: Data enters a Web application through an untrusted source, most frequently a web request.
The data is included in dynamic content that is sent to a web user without being validated for malicious code.
The malicious content sent to the web browser often takes the form of a segment of JavaScript, but may also include HTML, Flash or any other type of code that the browser may execute.
The variety of attacks based on XSS is almost limitless, but they commonly include transmitting private data like cookies or other session information to the attacker, redirecting the victim to web content controlled by the attacker, or performing other malicious operations on the user’s machine under the guise of the vulnerable site.

From: https://www.owasp.org/index.php/Cross-site_Scripting_(XSS)