Strict-Transport-Security header - HTTP | MDN

The HTTP Strict-Transport-Security response header (often abbreviated as HSTS) informs browsers that the host should only be accessed using HTTPS, and that any future attempts to access it using HTTP should automatically be upgraded to HTTPS. Additionally, on future connections to the host, the browser will not allow the user to bypass secure connection errors, such as an invalid certificate. HSTS identifies a host by its domain name only.