How should I use a bootstrap when I have already coded a front end of the website using CSS 3?

Answer 1

Assuming you mean the twitter bootstrap css framework, the first question would be why you want to include a CSS framework at all when you already created your own CSS.

I can think of two reasons:

1. You want to include additional CSS e.g. to use the grid-system
2. You want to include certain components requiring bootstraps javascript

In the first case, you will likely experience some collisions with your own CSS. To minimize this issue, make sure to include bootstrap's css prior to your own. But make sure you don't unintentionally overwrite bootstrap declarations you might want to keep (e.g. if you declared a .container yourself).

If this is just about JS, you might want to try only including the JS and then use the recommended markup and design it yourself. However, it is likely that you have to backward engineer a few classes the JS might apply to elements if certain triggers happen.

Make sure not to forget about jQuery & popper! Both are required for some functionality. The current bootstrap docs provide a list of what particular elements require the libraries (in the introduction)