How to get all posts from a category relation?

Hi, I’m trying to create a blog about Front-End development.

I managed to get all the posts with : http://localhost:1337/api/posts

I managed to get all the categories with : http://localhost:1337/api/categories

But now I want to get all the posts from the CSS or JavaScript category for example.
The Posts and Categories are linked by a relation.

I looked at the Strapi’s populate documentation : here

But I couldn’t make it, I’m confused with the “level” and everything about the population topic.
Here is what I tried so far :

http://localhost:1337/api/posts?populate[0]=css // it returns all the posts
http://localhost:1337/api/posts?populate[1] // error
http://localhost:1337/api/posts?populate[CSS] // error

Thanks for the hand!