Kind of strange to call it browserless as though Skia requires one to begin with. Been using the .NET Skia library in CLI and desktop applications for a while.
They're not saying Skia itself is browserless; they're saying that "Skia Canvas" is a non-browser-based implementation of what people would normally call the browser Canvas API.
Canvas is one of the easiest browser APIs to implement because most of it maps 1:1 to PostScript-style 2D APIs.
It was created by Apple as a wrapper for CoreGraphics back around Mac OS X Tiger 10.4. (IIRC, Canvas even debuted in Apple's Dashboard widgets rather than Safari? I'm not 100% sure.)
I remember writing my own Canvas implementation around 2008-2009 using JavaScriptCore and CoreGraphics. It took maybe two days. I used it for a video application where JS plugins needed to render 2D graphics but I couldn't provide a browser engine because that would have been too slow and unpredictable for render functions that get called in the video render loop.
I'm not claiming I implemented everything from scratch. Since it was a Mac OS X Cocoa app, it would just call the APIs available on that platform, like Apple's original Canvas implementation in WebKit.
Also since this was a video application running custom scripts written specifically for that host app, there wasn't a big worry about exact compatibility with any other Canvas-using code.
Anyway, here's the old code that doesn't look like there's anything interesting going on:
My previous startup Vidmaker created a browser-based collaborative video editor (think Google Docs for iMovie) in 2014 which did live preview in the browser and final render server-side. To use the same typescript-based "engine" for both we re-implemented most of the media browser APIs in NodeJS: 2d and 3d canvas, WebAudio, image APIs, something akin to HTML Video, etc. It worked amazingly well.
We got acqui-hired in 2015 and the product was shut down, but in hindsight I wish we'd open sourced at least those libraries.
Automattic acquired LearnBoost who employed TJ Holowaychuk, the original creator of node-canvas (and also Express and tons else). Automattic has not been involved in any way since at least 2017 when I joined node-canvas.