diff --git a/emcc.c b/emcc.c
index 62ea9f8..3c9850a 100644
--- a/emcc.c
+++ b/emcc.c
@@ -260,9 +260,10 @@ void mousemove(int x, int y, int buttons)
 }
 
 /*
- * Keyboard handler called from JS.
+ * Keyboard handler called from JS.  Returns true if the key was
+ * handled and hence the keydown event should be cancelled.
  */
-void key(int keycode, const char *key, const char *chr, int location,
+bool key(int keycode, const char *key, const char *chr, int location,
          bool shift, bool ctrl)
 {
     /* Key location constants from JavaScript. */
@@ -366,7 +367,9 @@ void key(int keycode, const char *key, const char *chr, int location,
 
         midend_process_key(me, 0, 0, keyevent);
         update_undo_redo();
+        return true; /* We've probably handled the event. */
     }
+    return false; /* Event not handled, because we don't even recognise it. */
 }
 
 /*
diff --git a/emccpre.js b/emccpre.js
index ee06019..37e5829 100644
--- a/emccpre.js
+++ b/emccpre.js
@@ -305,17 +305,17 @@ function initPuzzle() {
     };
 
     // Set up keyboard handlers. We call event.preventDefault()
-    // in the keydown handler. This means that
-    // while the canvas itself has focus, _all_ keypresses go only to
-    // the puzzle - so users of this puzzle collection in other media
+    // in the keydown handler if it looks like we might have
+    // done something with the key.  This means that users
+    // of this puzzle collection in other media
     // can indulge their instinct to press ^R for redo, for example,
     // without accidentally reloading the page.
-    key = Module.cwrap('key', 'void', ['number', 'string', 'string',
-                                       'number', 'number', 'number']);
+    key = Module.cwrap('key', 'boolean', ['number', 'string', 'string',
+                                          'number', 'number', 'number']);
     onscreen_canvas.onkeydown = function(event) {
-        key(event.keyCode, event.key, event.char, event.location,
-            event.shiftKey ? 1 : 0, event.ctrlKey ? 1 : 0);
-        event.preventDefault();
+        if (key(event.keyCode, event.key, event.char, event.location,
+                event.shiftKey ? 1 : 0, event.ctrlKey ? 1 : 0))
+            event.preventDefault();
     };
 
     // command() is a C function called to pass back events which