> Why is Firefox's CSS styling engine not already "in" the browser?

It is, Firefox can do CSS and styles just now. Stylo is a replacement for that.

> To me it seems like Stylo is just a parallelized replacement for the old system, Servo. What am I missing here?

Servo isn’t the old system. Servo is a research project to build a layout engine (analogous to Gecko which Firefox currently uses) in Rust, of which Stylo is a part.

> Also, why are you celebrating an alternative for WebKit? Isn't it already supported by most major browsers?

It sounds like you’re confused about what WebKit is. WebKit and its forks are used by a number of browsers (Chrome, Safari etc.).

EdgeHTML, Gecko and WebKit are, broadly speaking, competitors which attempt to implement certain standards and frequently disagree about how to do so. It’s important that there be alternatives to WebKit with significant market share because otherwise how WebKit implements a feature becomes the de facto reference implementation.

Thanks to you (and all the other commenters) for the detailed answers! I learned a lot!

Servo is a complete (although WIP) browser engine in Rust. Stylo is a project to plug the style parts of Servo into Firefox, which currently uses the Gecko browser engine. This is part of Mozilla's ongoing efforts to modernize Firefox by gradually rewriting it in Rust.

WebKit is technically only used by Safari. Chrome uses a WebKit fork called Blink, and MS Edge uses its own engine called Chakra.

I assume the above commenter was expressing relief that someone besides Google is making a fast, modern web browser these days.

Chakra is just the JS implementation in Edge; it's the equivalent of JavaScriptCore in Safari, V8 in Chrome, or SpiderMonkey in Firefox.

The layout engine bits in Edge are https://en.wikipedia.org/wiki/EdgeHTML

Servo is not Firefox's browser engine. Gecko is. Servo is an experimental highly-parallel browser engine in Rust. It's a separate project.

Stylo takes Servo's styling engine and puts it inside Gecko.