Website administrators and security professionals can submit their domains to the HSTS Preload List, a list of sites that are hardcoded into browsers to enforce HTTPS connections. This service helps protect against protocol downgrade attacks and ensures a secure browsing experience.
HTTP Strict Transport Security (HSTS) is a mechanism that allows websites to instruct browsers to only access the site over HTTPS. HSTS is supported by most major browsers and is defined in RFC6797.
HSTS preloading provides an additional layer of security by ensuring that browsers will only connect to the site over HTTPS, even on the first visit. This prevents on-path attackers from downgrading the connection to HTTP.
To be accepted into the HSTS Preload List, a site must:
max-age
of at least 31536000 seconds, includeSubDomains
directive, and preload
directive.