* Time zones and datetime processing between the server and multiple browsers in different time zones and locales, especially with daylight savings time. An offset is not a time zone. This is probably the #1 category of bugs I've had to fix in my career.
* Async issues and races, especially on slower connections (like how to combine different fetches in parallel in the UI when they all get sent at once but come back out of order)
* SVGs are basically tiny programs that happen to look like images. They're not just vector shapes.
* CSS animations. And flexbox hell. And CSS selector inheritance / priority overall.
* Basic computer science algorithms. I'm guilty of this too, having been a web dev for two decades but never learning what a binary search was until last year. I wouldn't be able to make a graph database or write the algos on my own.
* How to use the IDE debugger to step through code and watch variables etc. It's usually just a bunch of console logs
* Typescript generics, inheritance/extensions and more advanced features
* Haven't seen OOP in a good half decade or longer. It's all a bunch of scattered functions now.
* Basic graphics (Canvas, geometry, WebGL) and mapping (tiling, projections, cartography)
* Workers, server sent events, WASM
* CORS. God, I hate CORS.
* User-first design, instead of hostile adscape pop-up hell holes with exploding cookie jars and international surveillance
* Basic network stuff, like HTTP and DNS and caching (at least among the frontend folks)
* Basic UX/UI stuff, beyond just copying what the designer gives them without really understanding why
* Forms validation, autocomplete, error highlighting, client vs server validation, normalization (of phone numbers, addresses, etc.)
* (With React) States and contexts, now that lifecycle management isn't as important with functional components
* Next.js client vs server stuff and rehydration
* Mobile responsiveness and uncommon viewport sizes (this is STILL an issue!)
* Why we use frameworks and libs instead of raw HTML and vanilla JS
* Time zones, time zones, time zones. And CORS.