[padenot]

To paint broad strokes, the layout phase (~= take the HTML, take the CSS, determine the position and size of boxes) is largely sequential in production browser engine today. Selector matching (~= what CSS applies to what element) is parallel in Firefox today, via the Stylo Rust crate originally developed in the research browser engine Servo. Servo can do parallel layout in some capacity (but doesn't implement everything), https://github.com/servo/servo/wiki/Servo-Layout-Engines-Rep... is an interesting and recent document on the matter.

Parallel layout is generally considered to be a complex engineering problem by domain experts.

Rendering the page, as in deciding the colour of each pixel and putting them on the screen, based on the layout, style, and various other things, can be done with lots of parallelism, on the CPU or on the GPU (that is preferred on most platforms in production browser engines, these days).

https://hacks.mozilla.org/2017/08/inside-a-super-fast-css-en... is a really cool article that is related, that is a few years old but what it says is largely correct today.

[bloopernova]

Cool, thank you for the informative comment!

[nickcoury]

This 4 part series is related and excellent. https://developer.chrome.com/blog/inside-browser-part1/