Restrictions are crucial for several reasons: 1. Compatibility: Different email clients (like Gmail, Outlook, Yahoo Mail) render HTML emails differently, and restrictions help ensure consistent presentation. 2. Deliverability: Adhering to best practices can prevent emails from being marked as spam. 3. User Experience: A well-structured email is easier to read and interact with. 4. Legal Compliance: Regulations like the CAN-SPAM Act mandate certain elements in commercial emails.