diff --git a/README.md b/README.md
index 1f74a37..740fba4 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,10 @@ This polyfill will create a new style element for every `DocumentOrShadowRoot` i
 
 No changes will occur in a browser that supports the feature by default.
 
+## Requirements
+
+This polyfill should work as-is in modern browsers, however, to support IE11, you will also need to consume a polyfill for [ECMAScript `Symbol`s](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol). One popular and well supported option is [Core-JS](https://www.npmjs.com/package/core-js) which includes polyfills for many other modern features alongside a polyfill for `Symbol`.
+
 ## Installation
 
 The package can be installed either by copying the [polyfill file](./adoptedStyleSheets.js) or by [installing using npm](https://docs.npmjs.com/getting-started/).
diff --git a/adoptedStyleSheets.js b/adoptedStyleSheets.js
index c32dc3f..270dad3 100644
--- a/adoptedStyleSheets.js
+++ b/adoptedStyleSheets.js
@@ -39,7 +39,7 @@
     // Move style elements created before document.body
     // to the iframe along with future styles
     deferredStyleSheets.forEach(nativeStyleSheet => {
-      frameBody.append(nativeStyleSheet);
+      frameBody.appendChild(nativeStyleSheet);
       nativeStyleSheet.disabled = false;
     });
 
@@ -112,78 +112,79 @@
   // cannot be instantiated. The `new` operation will return the native
   // CSSStyleSheet object extracted from a style element appended to the
   // iframe.
-  class ConstructStyleSheet {
-    // Allows instanceof checks with the window.CSSStyleSheet.
-    static [Symbol.hasInstance](instance) {
-      return instance instanceof OldCSSStyleSheet;
+  function ConstructStyleSheet() {
+    // A style element to extract the native CSSStyleSheet object.
+    const basicStyleElement = document.createElement('style');
+
+    if (polyfillLoaded) {
+      // If the polyfill is ready, use the framebody
+      frameBody.appendChild(basicStyleElement);
+    } else {
+      // If the polyfill is not ready, move styles to head temporarily
+      document.head.appendChild(basicStyleElement);
+      basicStyleElement.disabled = true;
+      deferredStyleSheets.push(basicStyleElement);
     }
 
-    constructor() {
-      // A style element to extract the native CSSStyleSheet object.
-      const basicStyleElement = document.createElement('style');
+    const nativeStyleSheet = basicStyleElement.sheet;
 
-      if (polyfillLoaded) {
-        // If the polyfill is ready, use the framebody
-        frameBody.append(basicStyleElement);
-      } else {
-        // If the polyfill is not ready, move styles to head temporarily
-        document.head.append(basicStyleElement);
-        basicStyleElement.disabled = true;
-        deferredStyleSheets.push(basicStyleElement);
-      }
-
-      const nativeStyleSheet = basicStyleElement.sheet;
-
-      // A support object to preserve all the polyfill data
-      constructStyleSheetRegistry.set(nativeStyleSheet, {
-        adopters: new Map(),
-        actions: [],
-        basicStyleElement,
-      });
-
-      return nativeStyleSheet;
-    }
+    // A support object to preserve all the polyfill data
+    constructStyleSheetRegistry.set(nativeStyleSheet, {
+      adopters: new Map(),
+      actions: [],
+      basicStyleElement,
+    });
 
-    replace(contents) {
-      return new Promise((resolve, reject) => {
-        if (constructStyleSheetRegistry.has(this)) {
-          const {basicStyleElement} = constructStyleSheetRegistry.get(this);
-
-          basicStyleElement.innerHTML = contents;
-          resolve(basicStyleElement.sheet);
-          updateAdopters(this);
-        } else {
-          reject(
-            new DOMException(
-              "Failed to execute 'replace' on 'CSSStyleSheet': Can't call replace on non-constructed CSSStyleSheets.",
-              'NotAllowedError',
-            ),
-          );
-        }
-      });
-    }
+    return nativeStyleSheet;
+  }
 
-    replaceSync(contents) {
-      if (importPattern.test(contents)) {
-        throw new DOMException(
-          '@import rules are not allowed when creating stylesheet synchronously',
-          'NotAllowedError',
-        );
+  Object.defineProperty(ConstructStyleSheet, Symbol.hasInstance, {
+    get() {
+      return function(instance) {
+        return instance instanceof OldCSSStyleSheet;
       }
+    }
+  });
 
+  ConstructStyleSheet.prototype.replace = function(contents) {
+    return new Promise((resolve, reject) => {
       if (constructStyleSheetRegistry.has(this)) {
         const {basicStyleElement} = constructStyleSheetRegistry.get(this);
 
         basicStyleElement.innerHTML = contents;
+        resolve(basicStyleElement.sheet);
         updateAdopters(this);
-
-        return basicStyleElement.sheet;
       } else {
-        throw new DOMException(
-          "Failed to execute 'replaceSync' on 'CSSStyleSheet': Can't call replaceSync on non-constructed CSSStyleSheets.",
-          'NotAllowedError',
+        reject(
+          new DOMException(
+            "Failed to execute 'replace' on 'CSSStyleSheet': Can't call replace on non-constructed CSSStyleSheets.",
+            'NotAllowedError',
+          ),
         );
       }
+    });
+  }
+
+  ConstructStyleSheet.prototype.replaceSync = function(contents) {
+    if (importPattern.test(contents)) {
+      throw new DOMException(
+        '@import rules are not allowed when creating stylesheet synchronously',
+        'NotAllowedError',
+      );
+    }
+
+    if (constructStyleSheetRegistry.has(this)) {
+      const {basicStyleElement} = constructStyleSheetRegistry.get(this);
+
+      basicStyleElement.innerHTML = contents;
+      updateAdopters(this);
+
+      return basicStyleElement.sheet;
+    } else {
+      throw new DOMException(
+        "Failed to execute 'replaceSync' on 'CSSStyleSheet': Can't call replaceSync on non-constructed CSSStyleSheets.",
+        'NotAllowedError',
+      );
     }
   }
 
@@ -208,7 +209,7 @@
         const observer = observerRegistry.get(location);
 
         observer.disconnect();
-        newStyles.append(adoptedStyleElement);
+        newStyles.appendChild(adoptedStyleElement);
         observer.observe();
       } else {
         const clone = basicStyleElement.cloneNode(true);
@@ -217,13 +218,18 @@
         // element (e.g., it was disconnected).
         appliedActionsCursorRegistry.set(clone, 0);
         adopters.set(location, clone);
-        newStyles.append(clone);
+        newStyles.appendChild(clone);
       }
     }
 
     // Since we already removed all elements during appending them to the
     // document fragment, we can just re-add them again.
-    location.prepend(newStyles);
+    // we do this because IE11 doesn't support ParentNode.prepend
+    if (location.firstChild) {
+      location.insertBefore(newStyles, location.firstChild);
+    } else {
+      location.appendChild(newStyles);
+    }
 
     // We need to apply all actions we have done with the original CSSStyleSheet
     // to each new style element and to any other element that missed last