Google Online Security Blog: Broadening HSTS to secure more of the Web
The HSTS preload list is built in to all major browsers (Chrome, Firefox, Safari, Internet Explorer/Edge, and Opera). It consists of a list of hostnames for which browsers automatically enforce HTTPS-secured connections. For example, gmail.com is on the list, which means that the aforementioned browsers will never make insecure connections to Gmail; if the user types Gmail, the browser first changes it to Gmail before sending the request. This provides greater security because the browser never loads an http-to-https redirect page, which could be intercepted.
The HSTS preload list can contain individual domains or subdomains and even top-level domains (TLDs), which are added through the HSTS website. The TLD is the last part of the domain name, e.g., .com, .net, or .org. Google operates 45 TLDs, including .google, .how, and .soy. In 2015 we created the first secure TLD when we added .google to the HSTS preload list, and we are now rolling out HSTS for a larger number of our TLDs, starting with .foo and .dev.
The use of TLD-level HSTS allows such namespaces to be secure by default. Registrants receive guaranteed protection for themselves and their users simply by choosing a secure TLD for their website and configuring an SSL certificate, without having to add individual domains or subdomains to the HSTS preload list. Moreover, since it typically takes months between adding a domain name to the list and browser upgrades reaching a majority of users, using an already-secured TLD provides immediate protection rather than eventual protection. Adding an entire TLD to the HSTS preload list is also more efficient, as it secures all domains under that TLD without the overhead of having to include all those domains individually.
We hope to make some of these secure TLDs available for registration soon, and would like to see TLD-wide HSTS become the security standard for new TLDs.