HSTS is HTTP Strict Transport Security, which is a policy system for web sites to express a desire only to be contacted over HTTPS.
See https://github.com/chromium/hstspreload.org for the submission site code.
To check if a domain satisfies the requirements for preloading (assuming $PATH contains $GOPATH/bin/):
go install github.com/chromium/hstspreload/...@latest
hstspreload preloadabledomain wikipedia.org
For full documentation, see https://godoc.org/github.com/chromium/hstspreload