That is the best way to learn. It’s kinda like learning ruby before learning ruby on Rails. Or, learning JS before learning nodejs.
Well, should you learn CSS at all?
If you want to be a front end developer, you don’t need to know CSS, but since bootstrap is so easy to pick up, it good to know this framework to make your prototypes look nice.
If you want to be a front end designer it really doesn’t matter much whether you start with bootstrap or CSS. You should be familiar with Bootstrap or at least give clear reasons why not to use it.
Tbh, this question seems like an excuse to postpone learning one or the other subject.
When I tried to use Bootstrap with almost zero CSS knowledge it was totally confusing and unpredictable to me. I think, Bootstrap is easy when you already have some understanding how CSS itself works.
Plus, the framework is popular, so it’s elements will likely give your users deja vu feeling since they probably already saw those buttons, wells, icons and other design elements on many other sites.
A few questions to think about.
Will you advice people to learn Markup first and later HTML? (fill in your answer)
I totally disagree with the advice to learn bootstrap first. It is the easy way to make fancy web sites, I agree. But do you learn the basic skills of the web?
When people want to learn Java the advice is to learn basics of C first. There is a reason for this.
Nowadays you can find opinions about JQuery, another abstraction, and the added value compared to modern CSS3. Google these and make up your mind.