docs: document OffscreenCanvas (2.8)

runtime/reference/web_platform_apis.md

@@ -489,6 +489,55 @@ const worker = new Worker(import.meta.resolve("./worker.js"), {
 });
 ```
 
+## OffscreenCanvas
+
+Starting in Deno 2.8, the
+[`OffscreenCanvas`](https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas)
+API is available. `OffscreenCanvas` is a canvas that lives outside any DOM and
+can be used anywhere — including Web Workers — for off-thread rendering and
+image generation.
+
+### Supported rendering contexts
+
+`OffscreenCanvas#getContext` accepts two of the spec-defined context ids:
+
+- `"bitmaprenderer"` — returns an
+  [`ImageBitmapRenderingContext`](https://developer.mozilla.org/en-US/docs/Web/API/ImageBitmapRenderingContext)
+  for displaying an `ImageBitmap` produced via `createImageBitmap`.
+- `"webgpu"` — returns a
+  [`GPUCanvasContext`](https://developer.mozilla.org/en-US/docs/Web/API/GPUCanvasContext)
+  for rendering with WebGPU.
+
+Calling `getContext` with `"2d"`, `"webgl"`, or `"webgl2"` returns `null` —
+these contexts are not implemented in Deno.
+
+### Example: encoding an image to PNG
+
+Decode an image into an `ImageBitmap`, place it on an `OffscreenCanvas` via
+the `bitmaprenderer` context, and write the result to disk:
+
+```ts
+const data = await Deno.readFile("./input.jpg");
+const bitmap = await createImageBitmap(new Blob([data]));
+
+const canvas = new OffscreenCanvas(bitmap.width, bitmap.height);
+const ctx = canvas.getContext("bitmaprenderer")!;
+ctx.transferFromImageBitmap(bitmap);
+
+const blob = await canvas.convertToBlob({ type: "image/png" });
+await Deno.writeFile(
+  "./output.png",
+  new Uint8Array(await blob.arrayBuffer()),
+);
+```
+
+Typical uses:
+
+- producing thumbnails, format conversions, or social-card images at request
+  time without spinning up a headless browser,
+- running off-thread image work inside a Web Worker,
+- driving WebGPU rendering targets that don't need a window.
+
 ## Deviations of other APIs from spec
 
 ### Cache API