What should you test in client side code?

the act of testing client side code is easy for me but figuring out what to test is the hard part.

Some sources I read tested every single HTML element in a react component (to make sure it was rendered correctly). this can be tedious in you have a large application or large components. Some sources only tested components that are interactive or changes to the DOM

please can experienced testes please provide a guide as to what to test and what to avoid in a client side application and its individual components.

example would be helpful

thanks in advance :)