- An inability to work without frameworks. Lots of devs know React/Angular/Vue/whatever, but have a tenuous grasp of vanilla JavaScript

- Lack of computer science/algorithm knowledge. True of myself, and a lot of self taught developers.

- Lack of CSS knowledge. Quite a few devs know a decent amount of HTML, a decent amount of JavaScript (including one or two frameworks) but haven't brushed up on their CSS knowledge in a while. So they'll not know modern additions to the language, or in the worst cases, not know stuff like Grid or Flexbox.

- Accessibility in general is often a knowledge gap, especially given how many devs have an unfortunate mindset of 'every HTML element is a div'. Or thinking that buttons and links are interchangable.

- Browser compatibility is often downplayed a bit too. Quite a few devs know little about browser support for features, with some forgetting that browsers other than Chrome exist.

- And obviously, any sort of deeper knowledge of the tech stack or other aspects of system development. HTTP, server configuration, dev ops, etc are often a black box here.

* 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.

On it's own, this wouldn't be a problem of course. My problem with this is that there is no website on the web today that was technically impossible 5 years ago. Still, a 5 years old browser is unusable today due to this. (thinking primariy about ipads, which come with not updateable browsers)

Not sure if this is knowledge gap, or just simply unwillingness to write code that works on the widest range of devices...

for me, the highest degree of skill is when a site looks just awesome and at the same time can open in a browser from 20 years ago in exactly the same way and without errors as in a modern one