Vanilla JavaScript.
The biggest problem of development (as a whole, not just front end development) is the existence of frameworks. Most developers, especially freshmen and juniors, donʼt know what they deal with. They think in terms of DOM, bidirectional bindings, and all the like, instead of understanding how the language or the running environment (in this case, browsers) work.
You canʼt effectively harness frameworks if you donʼt understand the fundamentals of your job.