diff --git a/layout/base/nsIStyleSheetService.idl b/layout/base/nsIStyleSheetService.idl
index 637dba68816..dc14e2a0fde 100644
--- a/layout/base/nsIStyleSheetService.idl
+++ b/layout/base/nsIStyleSheetService.idl
@@ -45,15 +45,28 @@ interface nsIURI;
  * built-in list of user or agent style sheets.
  */
 
-[scriptable, uuid(c0f35b2d-de8f-4dab-9538-ff079bda74dc)]
+[scriptable, uuid(41d979dc-ea03-4235-86ff-1e3c090c5630)]
 interface nsIStyleSheetService : nsISupports
 {
   const unsigned long AGENT_SHEET = 0;
   const unsigned long USER_SHEET = 1;
 
   /**
-   * Synchrnously loads a style sheet from |sheetURI| and adds it to the list
+   * Synchronously loads a style sheet from |sheetURI| and adds it to the list
    * of user or agent style sheets.
    */
   void loadAndRegisterSheet(in nsIURI sheetURI, in unsigned long type);
+
+  /**
+   * Returns true if a style sheet at |sheetURI| has previously been
+   * added to the list of style sheets specified by |type|.
+   */
+  boolean sheetRegistered(in nsIURI sheetURI, in unsigned long type);
+
+  /**
+   * Remove the style sheet at |sheetURI| from the list of style
+   * sheets specified by |type|.  All documents loaded after
+   * this call will no longer use the style sheet.
+   */
+  void unregisterSheet(in nsIURI sheetURI, in unsigned long type);
 };
diff --git a/layout/base/nsStyleSheetService.cpp b/layout/base/nsStyleSheetService.cpp
index 8e0c1fb80c3..257d2481e57 100644
--- a/layout/base/nsStyleSheetService.cpp
+++ b/layout/base/nsStyleSheetService.cpp
@@ -37,8 +37,10 @@
  * ***** END LICENSE BLOCK ***** */
 
 #include "nsStyleSheetService.h"
+#include "nsIStyleSheet.h"
 #include "nsICSSLoader.h"
 #include "nsICSSStyleSheet.h"
+#include "nsIURI.h"
 #include "nsContentCID.h"
 #include "nsCOMPtr.h"
 #include "nsIServiceManager.h"
@@ -52,6 +54,7 @@ nsStyleSheetService *nsStyleSheetService::gInstance = nsnull;
 
 nsStyleSheetService::nsStyleSheetService()
 {
+  NS_ASSERTION(0 == AGENT_SHEET && 1 == USER_SHEET, "Invalid value for USER_SHEET or AGENT_SHEET");
   NS_ASSERTION(!gInstance, "Someone is using CreateInstance instead of GetService");
   gInstance = this;
 }
@@ -95,6 +98,24 @@ nsStyleSheetService::RegisterFromEnumerator(nsICategoryManager  *aManager,
   }
 }
 
+PRInt32
+nsStyleSheetService::FindSheetByURI(const nsCOMArray<nsIStyleSheet> &sheets,
+                                    nsIURI *sheetURI)
+{
+  for (PRInt32 i = sheets.Count() - 1; i >= 0; i-- ) {
+    PRBool bEqual;
+    nsCOMPtr<nsIURI> uri;
+    if (NS_SUCCEEDED(sheets[i]->GetSheetURI(getter_AddRefs(uri)))
+        && uri
+        && NS_SUCCEEDED(uri->Equals(sheetURI, &bEqual))
+        && bEqual) {
+      return i;
+    }
+  }
+
+  return -1;
+}
+
 nsresult
 nsStyleSheetService::Init()
 {
@@ -120,21 +141,40 @@ NS_IMETHODIMP
 nsStyleSheetService::LoadAndRegisterSheet(nsIURI *aSheetURI,
                                           PRUint32 aSheetType)
 {
+  NS_ENSURE_ARG(aSheetType == AGENT_SHEET || aSheetType == USER_SHEET);
+
   nsCOMPtr<nsICSSLoader> loader = do_CreateInstance(kCSSLoaderCID);
   nsCOMPtr<nsICSSStyleSheet> sheet;
   nsresult rv = loader->LoadAgentSheet(aSheetURI, getter_AddRefs(sheet));
   NS_ENSURE_SUCCESS(rv, rv);
 
-  switch (aSheetType) {
-  case AGENT_SHEET:
-    mAgentSheets.AppendObject(sheet);
-    break;
-  case USER_SHEET:
-    mUserSheets.AppendObject(sheet);
-    break;
-  default:
-    NS_ERROR("Unknown sheet type");
-  }
+  mSheets[aSheetType].AppendObject(sheet);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsStyleSheetService::SheetRegistered(nsIURI *sheetURI,
+                                     PRUint32 aSheetType, PRBool *_retval)
+{
+  NS_ENSURE_ARG(aSheetType == AGENT_SHEET || aSheetType == USER_SHEET);
+  NS_ENSURE_ARG_POINTER(sheetURI);
+  NS_PRECONDITION(_retval, "Null out param");
+
+  *_retval = (FindSheetByURI(mSheets[aSheetType], sheetURI) >= 0);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsStyleSheetService::UnregisterSheet(nsIURI *sheetURI, PRUint32 aSheetType)
+{
+  NS_ENSURE_ARG(aSheetType == AGENT_SHEET || aSheetType == USER_SHEET);
+  NS_ENSURE_ARG_POINTER(sheetURI);
+
+  PRInt32 foundIndex = FindSheetByURI(mSheets[aSheetType], sheetURI);
+  NS_ENSURE_TRUE(foundIndex >= 0, NS_ERROR_INVALID_ARG);
+  mSheets[aSheetType].RemoveObjectAt(foundIndex);
 
   return NS_OK;
 }
diff --git a/layout/base/nsStyleSheetService.h b/layout/base/nsStyleSheetService.h
index 6d8122bc2c6..16b896c592f 100644
--- a/layout/base/nsStyleSheetService.h
+++ b/layout/base/nsStyleSheetService.h
@@ -63,8 +63,8 @@ class nsStyleSheetService : public nsIStyleSheetService
 
   NS_HIDDEN_(nsresult) Init();
 
-  nsCOMArray<nsIStyleSheet>* AgentStyleSheets() { return &mAgentSheets; }
-  nsCOMArray<nsIStyleSheet>* UserStyleSheets() { return &mUserSheets; }
+  nsCOMArray<nsIStyleSheet>* AgentStyleSheets() { return &mSheets[AGENT_SHEET]; }
+  nsCOMArray<nsIStyleSheet>* UserStyleSheets() { return &mSheets[USER_SHEET]; }
 
   static nsStyleSheetService *gInstance;
 
@@ -75,8 +75,10 @@ class nsStyleSheetService : public nsIStyleSheetService
                                           nsISimpleEnumerator *aEnumerator,
                                           PRUint32             aSheetType);
 
-  nsCOMArray<nsIStyleSheet> mAgentSheets;
-  nsCOMArray<nsIStyleSheet> mUserSheets;
+  NS_HIDDEN_(PRInt32) FindSheetByURI(const nsCOMArray<nsIStyleSheet> &sheets,
+                                     nsIURI *sheetURI);
+
+  nsCOMArray<nsIStyleSheet> mSheets[2];
 };
 
 #endif