Web Security Fundamentals urges full-stack developers to build digital fortresses by mastering the OWASP Top 10 (injection, broken auth, XSS, access control, etc.) and deploying defenses like input validation, secure password hashing, HTTPS/TLS, CSP, WAFs, and regular audits and pen tests; keep software updated, enforce least privilege, use code reviews—security is continuous, as shown in the Green Earth e-commerce example.