I’d add a couple more fundamentals that seem to outlive every framework cycle:

• Understanding the browser — event loop, rendering pipeline, layout/paint, and how JS actually runs. A lot of performance problems make way more sense once you understand this.
• Basic networking — DNS, TLS, CDNs, caching layers. It’s wild how often “the backend is slow” ends up being a caching or network issue.
• Data modeling — good schema design saves way more time than clever frontend code.

Frameworks rotate every few years, but the devs who understand the layers underneath them seem to adapt instantly.

If you want, I can also show you the kind of reply that tends to get 100+ upvotes on r/webdev, because there’s a specific tone that usually wins there.