TLDR:
# Use 'always' so headers are also set for non-2XX and unset to avoid duplicates
<IfModule headers_module>
header unset Strict-Transport-Security
header always set Strict-Transport-Security "max-age=16070400; includeSubDomains;"
</IfModule>
Slightly1 longer version:
HTTPS everywhere is a worthwhile goal. Even when you have traffic that isn’t super interesting or sensitive by itself, the fact that you’re encrypting it makes traffic that really does need to be encrypted safer against tools that grab all of the encrypted traffic they can to decrypt later if/when possible.
One of the downsides of using HTTPS though is that without certain things in place, many users will still type domain.com in their address bar from time to time, completely missing out on the https://. While you can immediately redirect them, that very first request is a risk, since if a man-in-the-middle attack happens to catch that request, they can downgrade the entire connection.
Enter HTTP Strict Transport Security (HSTS). It’s a HTTP header that you can send on the first HTTPS connection you establish with a compatible client. Once you’ve done that, any further requests (until the header’s TTL expires without being renewed) will be sent to https:// no matter what the user types. Which solves the first request problem for all sessions… but it still doesn’t fix the very first time you have to get the header. So how do you fix that?