So to prevent a downgrade attack before a first connection is made, not only does the domain need to "includeSubdomains" - and have a valid lifetime (maybe of at least 31536000 seconds, or 1 year [this may just be a government standard]), but they'd also have to send the preload directive in their HSTS header and have been preloaded by that browser platform. If the domain is not preloaded, that first connection is required to get the HSTS information to the client in the Strict-Transport-Security header.