What Web Security Strategies are for the client side?

I am Computer Engineering and I am designing a software architecture for the client side. It's my first post, so it's likely to make some improper error.

I have been investigating the problems to which the user is exposed in certain web applications and I have found many, such as XSS (Cross-site scripting) or that the JavaScript execution environment is open and modifiable. Searching and searching I found some security strategies to follow: the isTrusted event property, the hash function and the content security policy (CSP). My question is if I am right after my research, and if you know any other strategy / technique to follow?