diff --git a/layout/base/nsCSSColorUtils.cpp b/layout/base/nsCSSColorUtils.cpp
index 3baca90cc0f..b0d9ad952b0 100644
--- a/layout/base/nsCSSColorUtils.cpp
+++ b/layout/base/nsCSSColorUtils.cpp
@@ -38,6 +38,7 @@
 /* functions that manipulate colors */
 
 #include "nsCSSColorUtils.h"
+#include "nsDebug.h"
 #include <math.h>
 
 // Weird color computing code stolen from winfe which was stolen
@@ -189,6 +190,12 @@ int NS_GetBrightness(PRUint8 aRed, PRUint8 aGreen, PRUint8 aBlue)
 
 PRInt32 NS_GetLuminosity(nscolor aColor)
 {
+  // When aColor is not opaque, the perceived luminosity will depend
+  // on what color(s) aColor is ultimately drawn on top of, which we
+  // do not know.
+  NS_ASSERTION(NS_GET_A(aColor) == 255,
+               "impossible to compute luminosity of a non-opaque color");
+  
   return (NS_GET_R(aColor) * RED_LUMINOSITY +
           NS_GET_G(aColor) * GREEN_LUMINOSITY +
           NS_GET_B(aColor) * BLUE_LUMINOSITY);
diff --git a/layout/base/nsCSSRendering.cpp b/layout/base/nsCSSRendering.cpp
index fc204b3f128..12e8c2dcf4d 100644
--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -1301,17 +1301,14 @@ nsCSSRendering::PaintBackground(nsPresContext* aPresContext,
     // If the window is intended to be opaque, ensure that we always
     // paint an opaque color for its root element, in case there's no
     // background at all or a partly transparent image.
-    //
-    // The default background color from the prescontext is under user
-    // control, so it might not be opaque either.
     nsIView* rView;
     vm->GetRootView(rView);
     if (!rView->GetParent() &&
         (!rView->HasWidget() ||
          rView->GetWidget()->GetTransparencyMode() == eTransparencyOpaque)) {
-      nscolor backColor =
-        NS_ComposeColors(NS_RGB(255,255,255),
-                         aPresContext->DefaultBackgroundColor());
+      nscolor backColor = aPresContext->DefaultBackgroundColor();
+      NS_ASSERTION(NS_GET_A(backColor) == 255,
+                   "default background color is not opaque");
 
       canvasColor.mBackgroundColor =
         NS_ComposeColors(backColor, canvasColor.mBackgroundColor);
diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index d0d9d0965d4..330a0fffdae 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -569,6 +569,11 @@ nsPresContext::GetDocumentColorPreferences()
                            mBackgroundColor);
   }
 
+  // Wherever we got the default background color from, ensure it is
+  // opaque.
+  mBackgroundColor = NS_ComposeColors(NS_RGB(0xFF, 0xFF, 0xFF),
+                                      mBackgroundColor);
+
   mUseDocumentColors = !useAccessibilityTheme &&
     nsContentUtils::GetBoolPref("browser.display.use_document_colors",
                                 mUseDocumentColors);
diff --git a/layout/generic/nsTextFrameThebes.cpp b/layout/generic/nsTextFrameThebes.cpp
index bcdbbe98746..4f57a677c6d 100644
--- a/layout/generic/nsTextFrameThebes.cpp
+++ b/layout/generic/nsTextFrameThebes.cpp
@@ -2968,7 +2968,13 @@ nsTextPaintStyle::InitCommonColors()
   const nsStyleBackground* bg =
     nsCSSRendering::FindNonTransparentBackground(sc);
   NS_ASSERTION(bg, "Cannot find NonTransparentBackground.");
-  mFrameBackgroundColor = bg->mBackgroundColor;
+
+  nscolor defaultBgColor = mPresContext->DefaultBackgroundColor();
+  NS_ASSERTION(NS_GET_A(defaultBgColor) == 255,
+               "default background color is not opaque");
+
+  mFrameBackgroundColor = NS_ComposeColors(defaultBgColor,
+                                           bg->mBackgroundColor);
 
   nsILookAndFeel* look = mPresContext->LookAndFeel();
   nscolor defaultWindowBackgroundColor, selectionTextColor, selectionBGColor;