docs: document Geometry Interfaces (DOMMatrix, etc.) (2.8)

runtime/reference/web_platform_apis.md

@@ -489,6 +489,34 @@ const worker = new Worker(import.meta.resolve("./worker.js"), {
 });
 ```
 
+## Geometry Interfaces
+
+Starting in Deno 2.8, the
+[Geometry Interfaces Module Level 1](https://drafts.fxtf.org/geometry/) types
+are available as globals. These are the same types you'd find in a browser:
+
+- [`DOMMatrix`](https://developer.mozilla.org/en-US/docs/Web/API/DOMMatrix) /
+  [`DOMMatrixReadOnly`](https://developer.mozilla.org/en-US/docs/Web/API/DOMMatrixReadOnly)
+  — 4×4 transform matrices for 2D and 3D operations.
+- [`DOMPoint`](https://developer.mozilla.org/en-US/docs/Web/API/DOMPoint) /
+  [`DOMPointReadOnly`](https://developer.mozilla.org/en-US/docs/Web/API/DOMPointReadOnly)
+  — points in 2D / 3D space.
+- [`DOMRect`](https://developer.mozilla.org/en-US/docs/Web/API/DOMRect) /
+  [`DOMRectReadOnly`](https://developer.mozilla.org/en-US/docs/Web/API/DOMRectReadOnly)
+  — axis-aligned rectangles.
+- [`DOMQuad`](https://developer.mozilla.org/en-US/docs/Web/API/DOMQuad) — a
+  quadrilateral defined by four points.
+
+```ts
+const m = new DOMMatrix().translateSelf(10, 20).scaleSelf(2);
+const p = new DOMPoint(1, 1).matrixTransform(m);
+console.log(p.x, p.y); // 12 22
+```
+
+These types are useful for graphics work — applying transforms to canvas
+drawings, computing layout math, or porting browser code that depends on
+geometry types.
+
 ## Deviations of other APIs from spec
 
 ### Cache API