HSTS is a mechanism that you can turn on for your website, and when someone visits your website, their browser (Chrome, Firefox, etc.) is automatically set to only display your website on HTTPS, the secure version, every time they visit your website.
It also follows that once you have HSTS enabled, you should have your site working properly on HTTPS, including a valid SSL certificate.
This mechanism can also be turned off in each browser for a specific domain. However, this must be done manually by each user for their browser, so it is only useful for your web development needs.
chrome://net-internals/#hsts in the address bar and remove
a specific domain using the form at the very bottom.
Close the Safari browser. Delete the file:
~/Library/Cookies/HSTS.plist. Try starting Safari again.