graphical output in code blocks

static/js/sandbox.js

@@ -0,0 +1,16 @@
+export class Sketch {
+    constructor(width, height) {
+        this.canvas = document.createElement("canvas");
+        this.canvas.width = width;
+        this.canvas.height = height;
+        this.ctx = this.canvas.getContext("2d");
+
+        document.body.appendChild(this.canvas);
+
+        postMessage({
+            kind: "resize",
+            width,
+            height,
+        });
+    }
+}