How to verify that your browser is securely connected to iCloud.com

Apple:

The iCloud website is protected with a digital certificate. If users get an invalid certificate warning in their browser while visiting www.icloud.com, they should pay attention to the warning and not proceed. Users should never enter their Apple ID or password into a website that presents a certificate warning. To verify that they are connected to the authentic iCloud website, users can check the contents of the digital certificate as shown below for Safari, Chrome, and Firefox—each of which provides both certificate information and warnings.