Strict-Transport-Security: max-age=31536000

HTTP Strict Transport Security (HSTS) is an opt-in security enhancement that is specified by a web application through the use of a special response header.

P3P: CP="This is not a P3P policy! See http://support.google.com/accounts/answer/151657?hl=en for more info."

Largely abandoned format for declaring website's privacy policy in machine-readable format. The only reason for many websites to use the header was that old versions of Microsoft Internet Explorer disallowed third-party cookies on websites missing P3P.

X-Content-Type-Options: nosniff

A non-standard but widely accepted header introduced originally by Microsoft to disable "content sniffing" or heuristic content type discovery in absence or mismatch of a proper HTTP Content-Type declaration, which led to a number of web attacks. In general, presence of the header with its only defined value of nosniff is considered as part of a properly secured HTTP response.

Server: YouTube Frontend Proxy

Announces web server software and optionally version details.

X-XSS-Protection: 0

Controls an Cross-Site Scripting (XSS) filters built into the majority of web browsers. The filter is usually turned on by default anyway, but requirement to set the header to 1 became part of canonical set of "secure" HTTP headers. Over time, vulnerabilities in the "sanitizing" mode filter were found, so 1; mode=block became the recommended value. Some companies decided that they don't really need a browser-side XSS filter to mess with their web services which are XSS-free anyway and they became consciously disabling the XSS filter by setting the header to 0.