> AFAIK, there is not yet a web browser core that can parallelize tasks in a meaningful way.

Firefox is getting there. The CSS style engine is parallelized already, and graphical compositing+rendering will follow shortly (already enabled experimentally in some configurations). Other parts of the browser will be next, including the DOM.

That sounds interesting. Any links to design documents or something similar?

The overall name of the project is Quantum but you have to dig in a bit to find out about the specific sub projects and how they work or plan to work.

Overview: https://wiki.mozilla.org/Quantum

CSS: https://hacks.mozilla.org/2017/08/inside-a-super-fast-css-en...

DOM: https://billmccloskey.wordpress.com/2016/10/27/mozillas-quan...

Rendering: https://www.masonchang.com/blog/2016/7/18/a-short-walkthroug...

Stylo shipped a long time ago now, in Firefox 57.

https://wiki.mozilla.org/Oxidation#Rust_Components has a list of what Rust has shipped within Firefox, though most of it has been motivated by safety guarantees rather than parallelism.

Thanks, Sam (!).

"Sam (!)" and a Swedish browser person… wonders who you are

Mozilla has been making pretty good progress with parallelization of firefox with their oxidation project. All browsers are splitting different tabs into different processes which means they are taking advantage of multithreading if you have more than one compute intensive tab/window open.

Splitting tabs into separate processes is mainstream these days, but it doesn't really help that much. You only need the focused tab to be snappy.

You only need the focused tab to be snappy, but to let it be snappy you need to keep all the other work away from that core. Gmail updating emails, youtube playing a video in the background, discord pinging chat messages at you, and so and and so forth.

With a modern javascript engine you can throttle the activity of tabs that are not focused. (And browsers do indeed do that. Pretty aggressively, too.)

I wish there was some extension that blocked any non-focused tabs from performing any javascript execution unless I whitelist the domain.

> It is what matters to web browsing performance.

Have a look at the web benchmarks though (starting at Mozilla Kraken 1.1):

https://www.anandtech.com/bench/product/2263?vs=2520

I have a crappy old Celeron 847 laptop, and my browsing experience is very much okay, not that different from even i3-8100. Yeah, I do sometimes hit very heavy sites, like bloomberg.com, but overall, experience is very much satisfactory.