## TL;DR
HSTS preload is a list browsers ship with that forces HTTPS for your domain from the very first visit. You opt in by sending a `Strict-Transport-Security` header with `preload` on your HTTPS responses, submitting at hstspreload.org, and waiting for the change to propagate. This is a one-way door, so get every subdomain on HTTPS first.

## The query
```
how to enable HSTS preload for your domain
```

## Use this when
- You run a production domain and want browsers to default to HTTPS before any redirect even happens
- You are closing out a TLS hardening checklist and HSTS preload is the last gap
- Your site had protocol downgrade or cookie-skimming concerns and you want the strongest browser-level guarantee
- You control the whole domain including all subdomains and can keep them on HTTPS forever

## Not for
- Sites that still serve any content over plain HTTP anywhere, even a forgotten subdomain
- Fixing mixed content, expired certs, or redirect loops; those get fixed separately
- Testing locally or on staging domains you might tear down; preload removal takes months
- Deciding between TLS providers or cert types; this is purely about the browser preload list

## Steps
1. Get all of HTTPS working everywhere on the domain. Every subdomain must serve HTTPS with valid certs, including www, mail, and internal ones you forgot about. Use a subdomain scanner to enumerate them. Expected output: a complete subdomain list with every entry loading over HTTPS.
2. Set the HSTS header with the preload flag. Your HTTPS responses should include `Strict-Transport-Security: max-age=63072000; includeSubDomains; preload`. Start with a small max-age for a week to catch mistakes before committing to two years. Expected output: `curl -sI https://example.com/` shows the header with your chosen max-age.
3. Verify the header is on every HTTPS response, not just the homepage. Check an API path, a 404 page, and at least one subdomain. Redirect chains from HTTP to HTTPS count too, the final HTTPS response must carry it. Expected output: the header is present everywhere you checked.
4. Submit at hstspreload.org. Enter your domain, fix whatever the checker complains about, and submit the form. Expected output: the checker shows all green and your submission is accepted.
5. Wait and confirm. Preload list updates roll out with browser releases, which takes weeks to months. After a few releases, check in a fresh browser profile that typing the bare domain goes straight to HTTPS with no HTTP hop. Expected output: fresh-profile test shows direct HTTPS, no plain HTTP request in devtools.
6. Remember removal is slow. If you ever need to drop out, you remove the header, request removal, and wait months for browsers to catch up. Expected output: this documented as accepted risk in your runbook before you opt in.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_p63cWESnO6RkOedT0U3qWQ