You can use alpine.js. There is even a way to integrate it with LV: https://dockyard.com/blog/2020/12/21/optimizing-user-experie...
Seriously? This has to be a joke.
You don't really need to use alpine either if you use Phoenix with Liveview, you can probably get away with just using vanilla js if all you really need is some small functionality of modals popping up etc.