What is the next big thing in frontend development?

I dream of browsers that have a setting to browse in "dark mode", i.e., if the user chooses so, the browser would provide by default dark background colours and white text colours for all websites. And CSS selectors that make it easy for web developers to configure the colours for dark and light modes.

Right now, it's up to every site to provide a dark theme, most don't bother, it's not straightforward to implement both, a light and a dark theme (even though CSS variables make it easier nowadays), and many times when the website provides a dark theme and the user selects it, the choice is stored in a cookie, which is far from ideal if you set your browser to remove all cookies on exit.

What I dream is that my browser has a setting that I can enable and will present all websites with a dark theme. The websites could use CSS to override the defaults for the dark mode, with something like this.

body {
    color: #111;
    background: #fff;
}

body:dark {
    color: #eee;
    background: #333;
}

And then the browser would use the styles with :dark when the user has the dark mode enabled, but they would get a basic default dark theme even if the CSS doesn't provide any :dark configuration.

I dream about a replacement for HTML CSS JS all together, they just have lots of crap for the sake of compatibility... I hope one day we will have clean version of all these langs once and for all.

I'm not sure whether I love or hate fully automatic code splitting.

I definitely see the edge being more and more powerful and custom, along with service workers.

Where code runs will be more and more important in this sense.

This is more of a full-stack response, but I'm very much looking forward to the time when GraphQL-as-a-service matures and you can code a SPA without ever configuring a database or a server.

At the moment, you can already do this in limited cases (AWS AppSync, Prisma, Hasura), but all the services have some glaring weak spots around relations. Somewhat surprisingly, the best one I've found is neo4j-graphql, which is only held back by a few bugs and the need to manage your own neo4j instance. Still, the ability to have a graphql api generated for you if you just define the models is very nice.

HTTP/2 everywhere

Probably someone will reinvent splitting the files, again, and in 5yrs a new webpack to glue them together again.

Meanwhile
My fav is CloudFlare Edge workers, they are a start to a new serverless revolution, but for front-end. I think more use cases will be found, for networking and backend services, and soon all CDN's providers will support this feature.

Modules in browsers - combined with TypeScript maybe the JS code will not look so bad.
PWA - after Apple joins the board, and hopefully market stores will have PWA apps too, it will open a new market.
WebAssembly ...

I hope for a Flutter web, Dart as a JS alternative sounds great.

A stricter separation of sites and apps.

Usage of less JS on websites, and if necessary, by progressive enhancement, so the pages work fine without it. Make the client work as little as possible. It's the most inclusive thing to do.

All while enabling a broader choice of technologies for more interactive non-vital web-applications and games, which I guess WASM is heading to. So Flash I guess, just not so battery-hungry and more secure.