diff --git a/web/app.js b/web/app.js
index e9ba568b6..5358e5810 100644
--- a/web/app.js
+++ b/web/app.js
@@ -169,21 +169,14 @@ let PDFViewerApplication = {
    * @private
    */
   async _readPreferences() {
-    // A subset of the Preferences that `AppOptions`, for compatibility reasons,
-    // is allowed to override if the `AppOptions` values matches the ones below.
-    const OVERRIDES = {
-      disableFontFace: true,
-      disableRange: true,
-      disableStream: true,
-      textLayerMode: TextLayerMode.DISABLE,
-    };
-
+    if (AppOptions.get('disablePreferences') === true) {
+      // Give custom implementations of the default viewer a simpler way to
+      // opt-out of having the `Preferences` override existing `AppOptions`.
+      return;
+    }
     try {
       const prefs = await this.preferences.getAll();
-      for (let name in prefs) {
-        if ((name in OVERRIDES) && AppOptions.get(name) === OVERRIDES[name]) {
-          continue;
-        }
+      for (const name in prefs) {
         AppOptions.set(name, prefs[name]);
       }
     } catch (reason) { }
diff --git a/web/app_options.js b/web/app_options.js
index 08fb28848..220b28455 100644
--- a/web/app_options.js
+++ b/web/app_options.js
@@ -58,6 +58,9 @@ const defaultOptions = {
     value: false,
     kind: OptionKind.VIEWER,
   },
+  /**
+   * The `disablePreferences` is, conditionally, defined below.
+   */
   enablePrintAutoRotate: {
     /** @type {boolean} */
     value: false,
@@ -218,6 +221,11 @@ const defaultOptions = {
 };
 if (typeof PDFJSDev === 'undefined' ||
     PDFJSDev.test('!PRODUCTION || GENERIC')) {
+  defaultOptions.disablePreferences = {
+    /** @type {boolean} */
+    value: false,
+    kind: OptionKind.VIEWER,
+  };
   defaultOptions.locale = {
     /** @type {string} */
     value: (typeof navigator !== 'undefined' ? navigator.language : 'en-US'),
diff --git a/web/viewer.js b/web/viewer.js
index 8885162b2..2254731fc 100644
--- a/web/viewer.js
+++ b/web/viewer.js
@@ -203,6 +203,16 @@ function webViewerLoad() {
 
     window.PDFViewerApplication = pdfjsWebApp.PDFViewerApplication;
     window.PDFViewerApplicationOptions = pdfjsWebAppOptions.AppOptions;
+
+    if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('GENERIC')) {
+      // Give custom implementations of the default viewer a simpler way to
+      // set various `AppOptions`, by dispatching an event once all viewer
+      // files are loaded but *before* the viewer initialization has run.
+      const event = document.createEvent('CustomEvent');
+      event.initCustomEvent('webviewerloaded', true, true, {});
+      document.dispatchEvent(event);
+    }
+
     pdfjsWebApp.PDFViewerApplication.run(config);
   }
 }