diff --git a/index.html b/index.html
index 00ea511..8e5fd17 100644
--- a/index.html
+++ b/index.html
@@ -382,19 +382,6 @@
 
       if(new URLSearchParams(window.location.search).get('leporello') == null) {
         await import('./src/launch.js');
-        // TODO remove?
-        /*
-        const {init} = await import('./src/index.js');
-        (
-          document.readyState == 'complete'
-            ? Promise.resolve()
-            : new Promise(resolve => 
-                window.addEventListener('load', resolve)
-              )
-        ).then(() => {
-          init(document.getElementById('app'))
-        })
-        */
       }
     </script>
   </head>
diff --git a/service_worker.js b/service_worker.js
index 6867862..3d230f7 100644
--- a/service_worker.js
+++ b/service_worker.js
@@ -29,21 +29,38 @@ self.addEventListener('message', async function(e) {
   e.ports[0].postMessage(reply)
 })
 
-// Fake URL base prepended by code responsible for module loading
-const FAKE_URL_BASE = 'https://leporello.import/'
+// Fake directory, http requests to this directory intercepted by service_worker
+const FILES_ROOT = '__leporello_files'
 
 self.addEventListener("fetch", event => {
-  if(event.request.url.startsWith(FAKE_URL_BASE)) {
-    if(dir_handle != null) {
-      const headers = new Headers([
-        ['Content-Type', 'text/javascript']
-      ])
-      const path = event.request.url.replace(FAKE_URL_BASE, '')
-      const response = read_file(dir_handle, path).then(file => 
-        new Response(file, {headers})
-      )
-      event.respondWith(response)
+  const url = new URL(event.request.url)
+  if(url.pathname.startsWith('/' + FILES_ROOT)) {
+    const path = url.pathname.replace('/' + FILES_ROOT + '/', '')
+
+    let file
+
+    if(path == '__leporello_blank.html') {
+      file = Promise.resolve('')
+    } else if(dir_handle != null) {
+      file = read_file(dir_handle, path)
+    } else {
+      // Delegate request to browser
+      return
     }
+
+    const headers = new Headers([
+      [
+        'Content-Type', 
+        path.endsWith('.js') || path.endsWith('.mjs')
+          ? 'text/javascript'
+          : 'text/html'
+      ]
+    ])
+
+    const response = file.then(file => 
+      new Response(file, {headers})
+    )
+    event.respondWith(response)
   }
 })
 
diff --git a/src/editor/ui.js b/src/editor/ui.js
index 876968b..6880711 100644
--- a/src/editor/ui.js
+++ b/src/editor/ui.js
@@ -227,7 +227,7 @@ export class UI {
               state.html_file == f
                 ? { value: f, selected: true }
                 : { value: f},
-              f == '' ? 'abount:blank' : f
+              f == '' ? 'about:blank' : f
             )
           )
       ),
diff --git a/src/effects.js b/src/effects.js
index 772b4ae..e418304 100644
--- a/src/effects.js
+++ b/src/effects.js
@@ -6,7 +6,7 @@ import {
   get_async_calls
 } from './calltree.js'
 import {FLAGS} from './feature_flags.js'
-import {exec} from './index.js'
+import {exec, FILES_ROOT} from './index.js'
 
 // Imports in the context of `run_window`, so global variables in loaded
 // modules refer to that window's context 
@@ -26,9 +26,12 @@ const load_external_imports = async state => {
       /^\w+:\/\//.test(u)    
         ? // starts with protocol, import as is
           u
-        : //local path, load using File System Access API, see service_worker.js
-          // Append fake host that will be intercepted in service worker
-          'https://leporello.import/' + u
+        : // local path, load using File System Access API, see service_worker.js
+          // Append special URL segment that will be intercepted in service worker
+          // Note that we use the same origin as current page (where Leporello
+          // is hosted), so Leporello can access window object for custom
+          // `html_file`
+          window.location.origin + '/' + FILES_ROOT + '/' + u
     ))
   )
   const modules = Object.fromEntries(
diff --git a/src/index.js b/src/index.js
index db46492..81373e2 100644
--- a/src/index.js
+++ b/src/index.js
@@ -8,6 +8,9 @@ const EXAMPLE = `const fib = n =>
     : fib(n - 1) + fib(n - 2)
 fib(6)`
 
+// Fake directory, http requests to this directory intercepted by service_worker
+export const FILES_ROOT = '__leporello_files'
+
 const set_error_handler = w => {
   // TODO err.message
   w.onerror = (msg, src, lineNum, colNum, err) => {
@@ -19,9 +22,10 @@ const set_error_handler = w => {
 }
 
 const get_html_url = state => {
+  const base = window.location.origin + '/' + FILES_ROOT + '/' 
   return state.html_file == ''
-    ? 'about:blank'
-    : window.location.origin + '/' + state.html_file + '?leporello'
+    ? base + '__leporello_blank.html'
+    : base + state.html_file + '?leporello'
 }
 
 // By default run code in hidden iframe, until user explicitly opens visible