Trust website certificate in macOS

Valid use cases I came across for doing this:

1. Enabling HTTPS for router's LAN web access.

Steps:

• Go to the website in Safari.
• There will be a prompt alert about the website's certificate.
• Click on "Show certificate".
• Drag and drop the certificate icon and place it somewhere on your computer.
• Double-click the certificate to add it to the System's Keychain.
• Search for the certificate in the "Keychain Access" app.
• Expand the Trust section and mark everything as "Always Trust".