Replace the <script> solution by an Image() solution

Create an <img> element; its error event will be fired asynchronously.

This solution is a non-intrusive alternative to the <script> solution, because it doesn't need to add a new element to the document.

README.md

@@ -44,9 +44,9 @@ Note that Internet Explorer 8 includes a synchronous version of `postMessage`. W
 
 Unfortunately, `postMessage` has completely different semantics inside web workers, and so cannot be used there. So we turn to [`MessageChannel`][MessageChannel], which has worse browser support, but does work inside a web worker.
 
-### `<script> onreadystatechange`
+### `<img> onerror`
 
-For our last trick, we pull something out to make things fast in Internet Explorer versions 6 through 8: namely, creating a `<script>` element and firing our calls in its `onreadystatechange` event. This does execute in a future turn of the event loop, and is also faster than `setTimeout(…, 0)`, so hey, why not?
+For our last trick, we pull something out to make things fast in Internet Explorer versions 6 through 8: namely, creating a `<img>` element with an invalid `src`, and firing our calls in its `onerror` event. This does execute in a future turn of the event loop, and is also faster than `setTimeout(…, 0)`, so hey, why not?
 
 ## Usage
 

setImmediate.js

@@ -118,20 +118,18 @@
         };
     }
 
-    function installReadyStateChangeImplementation() {
-        var html = doc.documentElement;
+    function installImageImplementation() {
         setImmediate = function() {
             var handle = addFromSetImmediateArguments(arguments);
-            // Create a <script> element; its readystatechange event will be fired asynchronously once it is inserted
-            // into the document. Do so, thus queuing up the task. Remember to clean up once it's been called.
-            var script = doc.createElement("script");
-            script.onreadystatechange = function () {
+            var image = new global.Image();
+
+            image.onerror = function() {
                 runIfPresent(handle);
-                script.onreadystatechange = null;
-                html.removeChild(script);
-                script = null;
             };
-            html.appendChild(script);
+
+            // This will always cause an error event to fire.
+            image.src = "\0";
+
             return handle;
         };
     }
@@ -161,12 +159,12 @@
         // For web workers, where supported
         installMessageChannelImplementation();
 
-    } else if (doc && "onreadystatechange" in doc.createElement("script")) {
-        // For IE 6–8
-        installReadyStateChangeImplementation();
+    } else if (doc && global.Image) {
+        // For IE 6–8, maybe older browsers
+        installImageImplementation();
 
     } else {
-        // For older browsers
+        // Ultimate fallback
         installSetTimeoutImplementation();
     }