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!
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