diff --git a/client/src/client/frame-runner.js b/client/src/client/frame-runner.js
index 83b2c9babf..9ca9e2dae7 100644
--- a/client/src/client/frame-runner.js
+++ b/client/src/client/frame-runner.js
@@ -3,18 +3,12 @@ import jQuery from 'jquery';
 
 window.$ = jQuery;
 
-const testId = 'fcc-test-frame';
-if (window.frameElement && window.frameElement.id === testId) {
-  document.addEventListener('DOMContentLoaded', initTestFrame);
-}
-
-// For tests in CI.
 document.__initTestFrame = initTestFrame;
 
-async function initTestFrame() {
-  const code = (document.__source || '').slice(0);
-  if (!document.__getUserInput) {
-    document.__getUserInput = () => code;
+async function initTestFrame(e = {}) {
+  const code = (e.code || '').slice(0);
+  if (!e.getUserInput) {
+    e.getUserInput = () => code;
   }
 
   /* eslint-disable no-unused-vars */
@@ -43,7 +37,7 @@ async function initTestFrame() {
   /* eslint-enable no-unused-vars */
 
   let Enzyme;
-  if (document.__loadEnzyme) {
+  if (e.loadEnzyme) {
     let Adapter16;
     /* eslint-disable no-inline-comments */
     [{ default: Enzyme }, { default: Adapter16 }] = await Promise.all([
@@ -66,7 +60,7 @@ async function initTestFrame() {
       // eslint-disable-next-line no-eval
       const test = eval(testString);
       if (typeof test === 'function') {
-        await test(document.__getUserInput);
+        await test(e.getUserInput);
       }
       return { pass: true };
     } catch (err) {
@@ -81,7 +75,4 @@ async function initTestFrame() {
       };
     }
   };
-
-  // notify that the window methods are ready to run
-  document.__frameReady();
 }
diff --git a/client/src/client/workers/sass-compile.js b/client/src/client/workers/sass-compile.js
index 00a5ee7cd5..126a8618f0 100644
--- a/client/src/client/workers/sass-compile.js
+++ b/client/src/client/workers/sass-compile.js
@@ -1,5 +1,17 @@
-// eslint-disable-next-line no-undef
-importScripts('/js/sass.sync.js');
+// work around for SASS error in Edge
+// https://github.com/medialize/sass.js/issues/96#issuecomment-424386171
+if (!self.crypto) {
+  self.crypto = {
+    getRandomValues: function(array) {
+      for (var i = 0, l = array.length; i < l; i++) {
+        array[i] = Math.floor(Math.random() * 256);
+      }
+      return array;
+    }
+  };
+}
+
+self.importScripts('/js/sass.sync.js');
 
 self.onmessage = e => {
   const data = e.data;
@@ -11,3 +23,5 @@ self.onmessage = e => {
     }
   });
 };
+
+self.postMessage({ type: 'contentLoaded' });
diff --git a/client/src/client/workers/test-evaluator.js b/client/src/client/workers/test-evaluator.js
index 2478c52658..a74104cded 100644
--- a/client/src/client/workers/test-evaluator.js
+++ b/client/src/client/workers/test-evaluator.js
@@ -48,3 +48,5 @@ self.onmessage = async e => {
     }
   }
 };
+
+self.postMessage({ type: 'contentLoaded' });
diff --git a/client/src/templates/Challenges/components/CompletionModal.js b/client/src/templates/Challenges/components/CompletionModal.js
index b7c7908b7d..1ed0121e62 100644
--- a/client/src/templates/Challenges/components/CompletionModal.js
+++ b/client/src/templates/Challenges/components/CompletionModal.js
@@ -60,6 +60,44 @@ const propTypes = {
 };
 
 export class CompletionModal extends Component {
+  state = {
+    downloadURL: null
+  };
+
+  static getDerivedStateFromProps(props, state) {
+    const { files, isOpen } = props;
+    if (!isOpen) {
+      return null;
+    }
+    const { downloadURL } = state;
+    if (downloadURL) {
+      URL.revokeObjectURL(downloadURL);
+    }
+    let newURL = null;
+    if (Object.keys(files).length) {
+      const filesForDownload = Object.keys(files)
+        .map(key => files[key])
+        .reduce(
+          (allFiles, { path, contents }) => ({
+            ...allFiles,
+            [path]: contents
+          }),
+          {}
+        );
+      const blob = new Blob([JSON.stringify(filesForDownload, null, 2)], {
+        type: 'text/json'
+      });
+      newURL = URL.createObjectURL(blob);
+    }
+    return { downloadURL: newURL };
+  }
+
+  componentWillUnmount() {
+    if (this.state.downloadURL) {
+      URL.revokeObjectURL(this.state.downloadURL);
+    }
+  }
+
   render() {
     const {
       close,
@@ -67,22 +105,11 @@ export class CompletionModal extends Component {
       submitChallenge,
       handleKeypress,
       message,
-      files = {},
       title
     } = this.props;
     if (isOpen) {
       ga.modalview('/completion-modal');
     }
-    const showDownloadButton = Object.keys(files).length;
-    const filesForDownload = Object.keys(files)
-      .map(key => files[key])
-      .reduce(
-        (allFiles, { path, contents }) => ({
-          ...allFiles,
-          [path]: contents
-        }),
-        {}
-      );
     const dashedName = dasherize(title);
     return (
       <Modal
@@ -112,18 +139,17 @@ export class CompletionModal extends Component {
             bsStyle='primary'
             onClick={submitChallenge}
             >
-            Submit and go to next challenge <span className='hidden-xs'>(Ctrl + Enter)</span>
+            Submit and go to next challenge{' '}
+            <span className='hidden-xs'>(Ctrl + Enter)</span>
           </Button>
-          {showDownloadButton ? (
+          {this.state.downloadURL ? (
             <Button
               block={true}
               bsSize='lg'
               bsStyle='primary'
               className='btn-primary-invert'
               download={`${dashedName}.json`}
-              href={`data:text/json;charset=utf-8,${encodeURIComponent(
-                JSON.stringify(filesForDownload)
-              )}`}
+              href={this.state.downloadURL}
               >
               Download my solution
             </Button>
@@ -137,4 +163,7 @@ export class CompletionModal extends Component {
 CompletionModal.displayName = 'CompletionModal';
 CompletionModal.propTypes = propTypes;
 
-export default connect(mapStateToProps, mapDispatchToProps)(CompletionModal);
+export default connect(
+  mapStateToProps,
+  mapDispatchToProps
+)(CompletionModal);
diff --git a/client/src/templates/Challenges/utils/frame.js b/client/src/templates/Challenges/utils/frame.js
index c4fff6759b..ad118debb0 100644
--- a/client/src/templates/Challenges/utils/frame.js
+++ b/client/src/templates/Challenges/utils/frame.js
@@ -72,15 +72,24 @@ const buildProxyConsole = proxyLogger => ctx => {
   return ctx;
 };
 
-const writeTestDepsToDocument = frameReady => ctx => {
-  const { sources, loadEnzyme } = ctx;
-  // default for classic challenges
-  // should not be used for modern
-  ctx.document.__source = sources && 'index' in sources ? sources['index'] : '';
-  // provide the file name and get the original source
-  ctx.document.__getUserInput = fileName => toString(sources[fileName]);
-  ctx.document.__frameReady = frameReady;
-  ctx.document.__loadEnzyme = loadEnzyme;
+const initTestFrame = frameReady => ctx => {
+  const contentLoaded = new Promise(resolve => {
+    if (ctx.document.readyState === 'loading') {
+      ctx.document.addEventListener('DOMContentLoaded', resolve);
+    } else {
+      resolve();
+    }
+  });
+  contentLoaded.then(async() => {
+    const { sources, loadEnzyme } = ctx;
+    // default for classic challenges
+    // should not be used for modern
+    const code = sources && 'index' in sources ? sources['index'] : '';
+    // provide the file name and get the original source
+    const getUserInput = fileName => toString(sources[fileName]);
+    await ctx.document.__initTestFrame({ code, getUserInput, loadEnzyme });
+    frameReady();
+  });
   return ctx;
 };
 
@@ -107,7 +116,7 @@ export const createTestFramer = (document, frameReady, proxyConsole) =>
   flow(
     createFrame(document, testId),
     mountFrame(document),
-    writeTestDepsToDocument(frameReady),
+    writeContentToFrame,
     buildProxyConsole(proxyConsole),
-    writeContentToFrame
+    initTestFrame(frameReady)
   );
diff --git a/client/src/templates/Challenges/utils/worker-executor.js b/client/src/templates/Challenges/utils/worker-executor.js
index ed423c55ce..054a2754b0 100644
--- a/client/src/templates/Challenges/utils/worker-executor.js
+++ b/client/src/templates/Challenges/utils/worker-executor.js
@@ -10,9 +10,17 @@ class WorkerExecutor {
     this.getWorker = this.getWorker.bind(this);
   }
 
-  getWorker() {
+  async getWorker() {
     if (this.worker === null) {
-      this.worker = new Worker(`${this.location}${this.workerName}.js`);
+      this.worker = await new Promise((resolve, reject) => {
+        const worker = new Worker(`${this.location}${this.workerName}.js`);
+        worker.onmessage = e => {
+          if (e.data && e.data.type && e.data.type === 'contentLoaded') {
+            resolve(worker);
+          }
+        };
+        worker.onerror = e => reject(e.message);
+      });
     }
 
     return this.worker;
@@ -25,8 +33,8 @@ class WorkerExecutor {
     }
   }
 
-  execute(data, timeout = 1000) {
-    const worker = this.getWorker();
+  async execute(data, timeout = 1000) {
+    const worker = await this.getWorker();
     return new Promise((resolve, reject) => {
       // Handle timeout
       const timeoutId = setTimeout(() => {
diff --git a/curriculum/test/test-challenges.js b/curriculum/test/test-challenges.js
index e900bf4a4e..8697635052 100644
--- a/curriculum/test/test-challenges.js
+++ b/curriculum/test/test-challenges.js
@@ -291,11 +291,9 @@ async function createTestRunnerForDOMChallenge(
   await context.setContent(build);
   await context.evaluate(
     async(sources, loadEnzyme) => {
-      document.__source = sources && 'index' in sources ? sources['index'] : '';
-      document.__getUserInput = fileName => sources[fileName];
-      document.__frameReady = () => {};
-      document.__loadEnzyme = loadEnzyme;
-      await document.__initTestFrame();
+      const code = sources && 'index' in sources ? sources['index'] : '';
+      const getUserInput = fileName => sources[fileName];
+      await document.__initTestFrame({ code, getUserInput, loadEnzyme });
     },
     sources,
     loadEnzyme