diff --git a/gfx/thebes/gfxBlur.h b/gfx/thebes/gfxBlur.h
index 5031a68afc52..5c2a7f774f1c 100644
--- a/gfx/thebes/gfxBlur.h
+++ b/gfx/thebes/gfxBlur.h
@@ -102,6 +102,23 @@ public:
      */
     static gfxIntSize CalculateBlurRadius(const gfxPoint& aStandardDeviation);
 
+    /**
+     * Blurs a coloured rectangle onto aDestinationCtx. This is equivalent
+     * to calling Init(), drawing a rectangle onto the returned surface
+     * and then calling Paint, but may let us optimize better in the
+     * backend.
+     *
+     * @param aDestinationCtx      The destination to blur to.
+     * @param aRect                The rectangle to blur in device pixels.
+     * @param aCornerRadii         Corner radii for aRect, if it is a rounded
+     *                             rectangle.
+     * @param aBlurRadius          The standard deviation of the blur.
+     * @param aShadowColor         The color to draw the blurred shadow.
+     * @param aDirtyRect           An area in device pixels that is dirty and needs
+     *                             to be redrawn.
+     * @param aSkipRect            An area in device pixels to avoid blurring over,
+     *                             to prevent unnecessary work.
+     */
     static void BlurRectangle(gfxContext *aDestinationCtx,
                               const gfxRect& aRect,
                               gfxCornerSizes* aCornerRadii,
diff --git a/layout/base/nsCSSRendering.h b/layout/base/nsCSSRendering.h
index 9c9f4b06e8d6..623f047dddf8 100644
--- a/layout/base/nsCSSRendering.h
+++ b/layout/base/nsCSSRendering.h
@@ -807,6 +807,27 @@ public:
   static nsMargin GetBlurRadiusMargin(nscoord aBlurRadius,
                                       int32_t aAppUnitsPerDevPixel);
 
+  /**
+   * Blurs a coloured rectangle onto aDestinationCtx. This is equivalent
+   * to calling Init(), drawing a rectangle onto the returned surface
+   * and then calling DoPaint, but may let us optimize better in the
+   * backend.
+   *
+   * @param aDestinationCtx      The destination to blur to.
+   * @param aRect                The rectangle to blur in app units.
+   * @param aAppUnitsPerDevPixel The number of app units in a device pixel,
+   *                             for conversion.  Most of the time you'll
+   *                             pass this from the current PresContext if
+   *                             available.
+   * @param aCornerRadii         Corner radii for aRect, if it is a rounded
+   *                             rectangle.
+   * @param aBlurRadius          The blur radius in app units.
+   * @param aShadowColor         The color to draw the blurred shadow.
+   * @param aDirtyRect           The absolute dirty rect in app units. Used to
+   *                             optimize the temporary surface size and speed up blur.
+   * @param aSkipRect            An area in device pixels (NOT app units!) to avoid
+   *                             blurring over, to prevent unnecessary work.
+   */
   static void BlurRectangle(gfxContext* aDestinationCtx,
                             const nsRect& aRect,
                             int32_t aAppUnitsPerDevPixel,