Hackers use CSS tricks to bypass spam filters and track user activity. They leverage CSS properties like text-indent, opacity, and font-size to hide malicious content in emails, making it invisible to recipients while deceiving spam filters.

Additionally, attackers exploit CSS to track users by analyzing the support of different CSS rules in email clients. This allows them to determine the victim's operating system, screen size, and other device parameters, helping them craft targeted attacks.

To stay protected, experts recommend using advanced filters, privacy proxies, and increasing user awareness.