Why can't I collapse the menu into a sandwich?

Question

css

Greetings. I have a bootstrap menu, I want to collapse it into a sandwich when the screen is reduced. I can't figure out why it doesn't work.. Here's the fiddle .
In the description of the bootstrap there is this line:

Attention! The collapse plugin and the dynamic Bootstrap CSS file are required for the dynamic Navbar to work.

as I understand bootstrap-collapse.js should already be included in the build, but what is dynamic Bootstrap CSS? Is there anything else I don't have connected?

Answer 1

Forgot collapse class: jsfiddle.net/t2vgf5pb/3