1. Tester should understand the vulnerability of the system because by this skill it should be able to secure the system for the parts where the attacks can be done.
2. Understand the web technologies and communication techniques of them because the two way communication understanding is one of the most important skill a tester should have so that he/she should have a proper image of what the system doing and how it is doing it.
3. Ability yo code scripts and most importantly understand them because while testing there are many script code to be read and understand for any bug or error the system is facing.