Active Directory, Unable to login to webpage requiring AD account

I'm trying to login to a website that requires my Active Directory account. It's not working. Why?
Some resources that rely on the UOFI AD account authentication are computer logins, Exchange accounts, and restricted web sites. When you connect to an AD-authenticated web site, you will be prompted for your NetID and password. This authentication prompt is different in various web browsers as it may have either 2 or 3 empty fields, requiring you to type the domain name in differently:

If the login window contains two fields:

1. Type ad.uillinois.edu\netid (be sure to use backslash "\" (not "/") in the User Name field
2. Type your password

OR
1. Type netid@illinois.edu in the User Name field
2. Type your password

If the login window contains three fields: 

1. Type your NetID 
2. Type your password in the Password field 
3. Type ad.uillinois.edu in the domain field