As Google is encouraging use of HTTPS on whole website so its good time to force whole website on HTTPS, typically you can do it by a simple redirect from your web-server but now you can do it even more efficiently at client level by passing an additional HSTS header.
add_header Strict-Transport-Security "max-age=63072000; includeSubdomains; preload";
Header always set Strict-Transport-Security "max-age=63072000; includeSubdomains; preload"
This will tell the browser that the site should be accessible only via HTTPS so in-case you type http://yourdomain the browser itself redirect it to https://yourdomain (Once the header is received and browser stored the information locally for future use).
You can also submit your domain to include in HSTS Preloading list. This service will submit your domain for the inclusion in the Chrome list which is also followed by other major browsers. HSTS is supported by all mainstream browsers and you can find full compatibility matrix here. This list is hard-coded into browsers so the browser will automatically use HTTPS for domains included in that list.
So far all good so where is the CAUTION!
If you want to use HSTS then you must be sure that you will be supporting HTTPS on whole site for a long period and specially when you are submitting it for HSTS Preloading. Those lists are hard-coded in browsers and does not have any method to de-list your domain and even if you are able to submit the de-list request then it may take months to take effect as this involves approval of request, update list in newer version of browsers, release of newer version of browser etc, but this is still not enough until your visitors /users have the newer browser version installed.
So whenever your are going to use HSTS put special attention on “max-age”, “includeSubdomains”, “Preloading” and “HSTS Preload Submission” otherwise you will be in big trouble if you want to stop use of HSTS on your site or don’t want to use HTTP on any sub-domain etc, also if your certificate is bad i-e expired, bad_cert_domain etc then browser won’t allow you to bypass or proceed for HSTS enabled sites.