How to use CSS animations in Reactjs?

CSS animations in React.js can be used similarly as you use in regular HTML and CSS.

You can also use libraries such as react-transition-group and styled-components are often useful to handle more complex animations.

Here’s a simple example of how to use CSS animations in React.js:

1. First, you will need to create a CSS file with your animations.

```
/* style.css */
.fade-in { animation: fadeIn 1s;
}

@keyframes fadeIn { 0% { opacity: 0; } 100% { opacity: 1; }
}
```
1. Then, you can import this CSS file into your React component:
```
import ‘./style.css’;
```
1. Then you can apply this animation to any element in your render method by adding the class name:
```
render() { return (

I will fade in when this component mounts.

);
}
```
This will create a simple fade-in animation for the text.

However, for more complex animations, a robust solution such as react-spring or framer-motion is often a good choice. These libraries provide more advanced and performant solutions for animations, with react-spring being more physics-based and framer-motion being more timeline-based.

For conditional animations, or animations that need to respond to user input or component state, you might need to use inline styles or CSS-in-JS solutions that allow you to write your CSS within your JavaScript, giving you the full power of JavaScript to determine how and when your styles are applied.