I've started my journey with CSS when Bootstrap was pretty much the only relevant CSS framework; and can confirm. There was no VS Code (Notepad++, anyone?), but Chrome Dev Tools were very useful when trying to understand what's actually going on and that + trying to "Inspect Element" random websites actually taught me CSS.

Fun fact: border-radius wasn't supported well until one year (2010) before Bootstrap (2011) was first released. You had to do rounded corners with some annoying background-image hacks.