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.