diff --git a/docs/demo/python.md b/docs/demo/python.md
index ed339b0..a6a7a35 100644
--- a/docs/demo/python.md
+++ b/docs/demo/python.md
@@ -68,12 +68,14 @@ The
 module allows creating SVG images using simple drawing primitives. Rendering an
 image creates an output block displaying the image.
 
-`render(image, name='')`
+`render(image, name='') -> Future`
 
 - `image`: The image to be rendered.
 - `name`: The name of the output block. If a block with the same name already
   exists, it is replaced. Otherwise, a new block is added, keeping blocks
   ordered by name.
+- The returned future resolves to a tuple `(width, height)` that specifies the
+  size of the rendered image.
 
 ```{exec} python
 :name: python-graphics
@@ -117,22 +119,19 @@ import asyncio
 def saw(value, amplitude):
   return abs((value + amplitude) % (2 * amplitude) - amplitude)
 
+vx, vy, va = 101, 79, 181
+
 img = svg.Image(400, 100, stroke='red', style='width: 100%; height: 100%')
 g = img.group()
 paint_heart(g)
 
-margin = 20
-vx, vy, va = 101, 79, 181
-
 loop = asyncio.get_running_loop()
 start = loop.time()
 while True:
   t = loop.time() - start
-  x = margin + saw(t * vx, img.width - 2 * margin)
-  y = margin + saw(t * vy, img.height - 2 * margin)
-  a = (t * va) % 360.0
-  g.transform = svg.translate(x, y).rotate(a).scale(0.5)
-  render(img)
+  x, y, a = saw(t * vx, img.width), saw(t * vy, img.height), (t * va) % 360.0
+  g.transform = svg.translate(x, y).rotate(a)
+  img.width, img.height = await render(img)
   await asyncio.sleep(1 / 60)
 ```
 
diff --git a/tdoc/common/python/__init__.py b/tdoc/common/python/__init__.py
index 4f848f8..36f8c89 100644
--- a/tdoc/common/python/__init__.py
+++ b/tdoc/common/python/__init__.py
@@ -54,8 +54,9 @@ def write(self, data, /):
 
 @public
 def render(html, name=''):
+    """Render some HTML in an output block."""
     if not isinstance(html, str): html = ''.join(html)
-    js_render(run_id(), html, name)
+    return js_render(run_id(), html, name).then(lambda res: tuple(res))
 
 
 @public
diff --git a/tdoc/common/python/svg.py b/tdoc/common/python/svg.py
index 1cab589..867a281 100644
--- a/tdoc/common/python/svg.py
+++ b/tdoc/common/python/svg.py
@@ -4,7 +4,6 @@
 import html
 
 # TODO: Add <style> element
-# TODO: Make the output width accessible; maybe return it from render()
 
 def esc(v, quote=True):
     if not isinstance(v, str): v = str(v)
diff --git a/tdoc/common/static/tdoc-python.js b/tdoc/common/static/tdoc-python.js
index 719a673..299c334 100644
--- a/tdoc/common/static/tdoc-python.js
+++ b/tdoc/common/static/tdoc-python.js
@@ -198,7 +198,8 @@ class PythonExecutor extends Executor {
     }
 
     onRender(html, name) {
-        this.render(name, html);
+        const el = this.render(name, html);
+        return [el.scrollWidth, el.scrollHeight];
     }
 
     render(name, html) {
@@ -212,11 +213,11 @@ class PythonExecutor extends Executor {
         for (const el of this.output.children) {
             if (el.tdocName > name) {
                 el.before(new_el);
-                return;
+                return new_el;
             }
             if (el.tdocName === name) {
                 el.replaceWith(new_el);
-                return;
+                return new_el;
             }
         }
         this.output.appendChild(new_el);