StageXL does also use WebGL with a fallback to Canvas. Fortunately, most browsers supports WebGL nowadays.

Is there a feature comparison with Pixi somewhere? Why would one pick StageXL over Pixi, for example?

Sorry i missed your second question. I would choose StageXL over PixiJS because auf the Dart language. I also use TypeScript for other project but i prefer Dart. The Dart-to-JavaScript compiler does an awesome job. It not only transpiles Dart code to JavaScript (like the TypeScript compiler does for TypeScript) but it's a real compiler with great optimization and minification. The optimizations include inlining, dead code removal, hoisting, etc. The resulting JavaScript code is not pretty to look at, but pretty damned fast at runtime.

Thank you for both of your replies

Unfortunately no, but i'm not aware of any major feature that is not supported in comparision to Pixi.js. If you find something missing in StageXL i'm happy to receive a feature request :)