This is not a custom header (i.e. one with no UA semantics), it is a standardized header called X-Frame-Options which requires the “X-“ for browsers to recogniz

This is not a custom header (i.e. one with no UA semantics), it is a standardized header called X-Frame-Options which requires the “X-“ for browsers to recognize it. The prefix is an artifact of the era the header was introduced in. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-...