diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/src/PageStyle.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/src/PageStyle.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,127 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+* License, v. 2.0. If a copy of the MPL was not distributed with this file,
+* You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["PageStyle"];
+
+const Ci = Components.interfaces;
+
+/**
+ * The external API exported by this module.
+ */
+this.PageStyle = Object.freeze({
+  collect: function (docShell, frameTree) {
+    return PageStyleInternal.collect(docShell, frameTree);
+  },
+
+  restore: function (docShell, frameList, pageStyle) {
+    PageStyleInternal.restore(docShell, frameList, pageStyle);
+  },
+
+  restoreTree: function (docShell, data) {
+    PageStyleInternal.restoreTree(docShell, data);
+  }
+});
+
+// Signifies that author style level is disabled for the page.
+const NO_STYLE = "_nostyle";
+
+let PageStyleInternal = {
+  /**
+   * Collects the selected style sheet sets for all reachable frames.
+   */
+  collect: function (docShell, frameTree) {
+    let result = frameTree.map(({document: doc}) => {
+      let style;
+
+      if (doc) {
+        // http://dev.w3.org/csswg/cssom/#persisting-the-selected-css-style-sheet-set
+        style = doc.selectedStyleSheetSet || doc.lastStyleSheetSet;
+      }
+
+      return style ? {pageStyle: style} : null;
+    });
+
+    let markupDocumentViewer =
+      docShell.contentViewer.QueryInterface(Ci.nsIMarkupDocumentViewer);
+
+    if (markupDocumentViewer.authorStyleDisabled) {
+      result = result || {};
+      result.disabled = true;
+    }
+
+    return result && Object.keys(result).length ? result : null;
+  },
+
+  /**
+   * Restore the selected style sheet of all the frames in frameList
+   * to match |pageStyle|.
+   * @param docShell the root docshell of all the frames
+   * @param frameList a list of [frame, data] pairs, where frame is a
+   * DOM window and data is the session restore data associated with
+   * it.
+   * @param pageStyle the title of the style sheet to apply
+   */
+  restore: function (docShell, frameList, pageStyle) {
+    let disabled = pageStyle == NO_STYLE;
+
+    let markupDocumentViewer =
+      docShell.contentViewer.QueryInterface(Ci.nsIMarkupDocumentViewer);
+    markupDocumentViewer.authorStyleDisabled = disabled;
+
+    for (let [frame, data] of frameList) {
+      Array.forEach(frame.document.styleSheets, function(aSS) {
+        aSS.disabled = aSS.title && aSS.title != pageStyle;
+      });
+    }
+  },
+
+  /**
+   * Restores pageStyle data for the current frame hierarchy starting at the
+   * |docShell's| current DOMWindow using the given pageStyle |data|.
+   *
+   * Warning: If the current frame hierarchy doesn't match that of the given
+   * |data| object we will silently discard data for unreachable frames. We may
+   * as well assign page styles to the wrong frames if some were reordered or
+   * removed.
+   *
+   * @param docShell (nsIDocShell)
+   * @param data (object)
+   *        {
+   *          disabled: true, // when true, author styles will be disabled
+   *          pageStyle: "Dusk",
+   *          children: [
+   *            null,
+   *            {pageStyle: "Mozilla", children: [ ... ]}
+   *          ]
+   *        }
+   */
+  restoreTree: function (docShell, data) {
+    let disabled = data.disabled || false;
+    let markupDocumentViewer =
+      docShell.contentViewer.QueryInterface(Ci.nsIMarkupDocumentViewer);
+    markupDocumentViewer.authorStyleDisabled = disabled;
+
+    function restoreFrame(root, data) {
+      if (data.hasOwnProperty("pageStyle")) {
+        root.document.selectedStyleSheetSet = data.pageStyle;
+      }
+
+      if (!data.hasOwnProperty("children")) {
+        return;
+      }
+
+      let frames = root.frames;
+      data.children.forEach((child, index) => {
+        if (child && index < frames.length) {
+          restoreFrame(frames[index], child);
+        }
+      });
+    }
+
+    let ifreq = docShell.QueryInterface(Ci.nsIInterfaceRequestor);
+    restoreFrame(ifreq.getInterface(Ci.nsIDOMWindow), data);
+  }
+};