Right now you can paint the following targets: pressure (which makes the stroke thicker or thinner), colour, and its position (you can grab the stroke after it was drawn).
The UI is a shell written in React. The more interesting part is the renderer, it's coded in Rust and compiled to WASM and WebGPU. The performance is quite nice, and has room to improve even more. All input is routed through WASM so pointer events are really, really fast, stroke polling feels like a completely native app, there's no React sluggishness anywhere in the actual painting process. The brush overlays are also rendered in the GPU core so those are also smooth. Only the panels, sliders, etc. are React.