I’m currently working on bootstrap in Front End Development Libraries. I’m all fine doing those challenges, but when I wanted to test out them in CodePen, the codes doesn’t work. I’ve also tried pasting the solution into a blank page by double-clicking inspect element, it doesn’t have the result of bootstrap. It seems like bootstrap only works in FreeCodeCamp. Surprisingly, img-responsive works in CodePen only.
You should also be aware that there are changes to class names and components between versions. Make sure you refer to the docs for the version of Bootstrap you are using. The latest version 5 and the one taught in the curriculum (v3) are not the same.
You can look at the changelogs and migration sections to see what has changed between versions.