I wonder this first learning web development through a CSS framework myself. I started back in the early days of Sass wrestling with bootstrap layo...
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In my personal experience, I had used Bootstrap when I had started learning CSS. I had basic knowledge of CSS like how flex box and grids work, media queries and things like that. Using it made enabled me to make UIs faster without writing much css myself which helped me to practice and focus on building the webpage more. Gradually, and soon enough, I reached a point where I felt like I should have more control or should start exploring more designs and stuff which I could pick up faster since now I had better idea of how complex designs can be made and handled gracefully while keeping it responsive across devices.
I would say that it is perfectly fine for beginners to try out CSS libraries / frameworks so that they can focus on the ecosystem and not get tired writing everything from scratch right when they have started learning and developing! Once they start enjoying it and get more comfortable with everything, they can resume again with learning the details!
You don't need to have a deep understanding of CSS, at least with Bootstrap in my opinion. But knowing it will make you way more productive though if you known the concepts behind those utility classes like flexbox.
I think with any framework / library frontend or backend, it's best to have a good understanding of what the framework/library is doing behind the scenes before you use it. You wouldn't learn angular or react before having a great understanding of javaScript and you wouldn't learn laravel without having a solid grasp on PHP. I think the same applies for any CSS framework. Understand what everything's doing. You should be able to do it yourself without the help of something else. Use the tool to make your life easier. Not as a shortcut to avoid learning.
This is speaking from experience, I jumped on the bootstrap band wagon as soon as I learned about it and my CSS skills never really grew. It wasn't until I decided to not use frameworks for quite a few projects that I got to an advanced skill set with CSS/SASS. If I could go back and tell myself to do it differently I would.
I'd say don't use those at all, unless you just need to throw together an interface for something without visual aspiration.
So my answer would be: The ideal CSS-experience before using it is NONE. If you know CSS, build your own.
At the beginning I used CSS frameworks and Botstrap was easy way to get the code that you want, BUT I do not recommend it because making your own Css will make your life easy to understand and manipulate the code you wrote.
A very, very tiny minimum amount of CSS knowledge you need or none. As long as a CSS framework could covering all the necessary features to build a full-fledged UI. Maybe using CSS when to solve something tricky that a CSS framework couldn't do. Yea... You don't need it (CSS).
The only thing you need to know about CSS to work with frameworks is specificity, if you know this you can overwrite css rules