diff --git a/src/shared/compatibility.js b/src/shared/compatibility.js
index 3b2221f6f..86ec6e1d2 100644
--- a/src/shared/compatibility.js
+++ b/src/shared/compatibility.js
@@ -26,12 +26,10 @@ if ((typeof PDFJSDev === 'undefined' ||
 // users - https://github.com/mozilla/pdf.js/issues/9397
 if (typeof PDFJSDev === 'undefined' || !PDFJSDev.test('CHROME')) {
 
-var globalScope = require('./global_scope');
-
+const globalScope = require('./global_scope');
 const isNodeJS = require('./is_node');
 
 var userAgent = (typeof navigator !== 'undefined' && navigator.userAgent) || '';
-var isAndroid = /Android/.test(userAgent);
 var isIOSChrome = userAgent.indexOf('CriOS') >= 0;
 var isIE = userAgent.indexOf('Trident') >= 0;
 var isIOS = /\b(iPad|iPhone|iPod)(?=;)/.test(userAgent);
@@ -80,18 +78,6 @@ PDFJS.compatibilityChecked = true;
   }
 })();
 
-// Checks if navigator.language is supported
-// Support: IE<11
-(function checkNavigatorLanguage() {
-  if (typeof navigator === 'undefined') {
-    return;
-  }
-  if ('language' in navigator) {
-    return;
-  }
-  PDFJS.locale = navigator.userLanguage || 'en-US';
-})();
-
 // Support: Safari 6.0+, iOS
 (function checkRangeRequests() {
   // Safari has issues with cached range requests see:
@@ -103,25 +89,6 @@ PDFJS.compatibilityChecked = true;
   }
 })();
 
-// Support: Android, iOS
-(function checkCanvasSizeLimitation() {
-  if (isIOS || isAndroid) {
-    // 5MP
-    PDFJS.maxCanvasPixels = 5242880;
-  }
-})();
-
-// Disable fullscreen support for certain problematic configurations.
-// Support: IE11+ (when embedded).
-(function checkFullscreenSupport() {
-  if (!hasDOM) {
-    return;
-  }
-  if (isIE && window.parent !== window) {
-    PDFJS.disableFullscreen = true;
-  }
-})();
-
 // Provides document.currentScript support
 // Support: IE, Chrome<29.
 (function checkCurrentScript() {
diff --git a/web/app.js b/web/app.js
index e3128b118..a21b443f4 100644
--- a/web/app.js
+++ b/web/app.js
@@ -44,6 +44,7 @@ import { PDFThumbnailViewer } from './pdf_thumbnail_viewer';
 import { PDFViewer } from './pdf_viewer';
 import { SecondaryToolbar } from './secondary_toolbar';
 import { Toolbar } from './toolbar';
+import { viewerCompatibilityParams } from './viewer_compatibility';
 import { ViewHistory } from './view_history';
 
 const DEFAULT_SCALE_DELTA = 1.1;
@@ -561,7 +562,7 @@ let PDFViewerApplication = {
         support = false;
       }
     }
-    if (support && PDFJS.disableFullscreen === true) {
+    if (support && viewerCompatibilityParams['disableFullscreen'] === true) {
       support = false;
     }
 
diff --git a/web/ui_utils.js b/web/ui_utils.js
index bfff86ae5..9119147cb 100644
--- a/web/ui_utils.js
+++ b/web/ui_utils.js
@@ -65,14 +65,6 @@ let NullL10n = {
   },
 };
 
-/**
- * Disables fullscreen support, and by extension Presentation Mode,
- * in browsers which support the fullscreen API.
- * @var {boolean}
- */
-PDFJS.disableFullscreen = (PDFJS.disableFullscreen === undefined ?
-                           false : PDFJS.disableFullscreen);
-
 /**
  * Enables CSS only zooming.
  * @var {boolean}
@@ -110,7 +102,7 @@ if (typeof PDFJSDev === 'undefined' ||
    */
   PDFJS.locale =
     (PDFJS.locale === undefined && typeof navigator !== 'undefined' ?
-     navigator.language : PDFJS.locale);
+     navigator.language : PDFJS.locale) || 'en-US';
 }
 
 /**
diff --git a/web/viewer_compatibility.js b/web/viewer_compatibility.js
new file mode 100644
index 000000000..e6aadea80
--- /dev/null
+++ b/web/viewer_compatibility.js
@@ -0,0 +1,44 @@
+/* Copyright 2018 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+let compatibilityParams = Object.create(null);
+if (typeof PDFJSDev === 'undefined' || PDFJSDev.test('GENERIC')) {
+  const userAgent =
+    (typeof navigator !== 'undefined' && navigator.userAgent) || '';
+  const isAndroid = /Android/.test(userAgent);
+  const isIE = /Trident/.test(userAgent);
+  const isIOS = /\b(iPad|iPhone|iPod)(?=;)/.test(userAgent);
+
+  // Disable fullscreen support for certain problematic configurations.
+  // Support: IE11+ (when embedded).
+  (function checkFullscreenSupport() {
+    if (isIE && window.parent !== window) {
+      compatibilityParams.disableFullscreen = true;
+    }
+  })();
+
+  // Limit canvas size to 5 mega-pixels on mobile.
+  // Support: Android, iOS
+  (function checkCanvasSizeLimitation() {
+    if (isIOS || isAndroid) {
+      compatibilityParams.maxCanvasPixels = 5242880;
+    }
+  })();
+}
+const viewerCompatibilityParams = Object.freeze(compatibilityParams);
+
+export {
+  viewerCompatibilityParams,
+};