Use Shadow DOM to display fallback content for images

LayoutTests/platform/linux/tables/mozilla/bugs/bug2479-1-expected.txt

Index: LayoutTests/platform/linux/tables/mozilla/bugs/bug2479-1-expected.txt
diff --git a/LayoutTests/platform/linux/tables/mozilla/bugs/bug2479-1-expected.txt b/LayoutTests/platform/linux/tables/mozilla/bugs/bug2479-1-expected.txt
index 96ff9565350f968c3652257b965fa25866833b28..ab86c2778fddb1b7b43809b6f0281b4a61351de7 100644
--- a/LayoutTests/platform/linux/tables/mozilla/bugs/bug2479-1-expected.txt
+++ b/LayoutTests/platform/linux/tables/mozilla/bugs/bug2479-1-expected.txt
@@ -274,7 +274,7 @@ layer at (0,0) size 785x989
       RenderBlock {HR} at (0,825.69) size 769x2 [border: (1px inset #EEEEEE)]
       RenderBlock {P} at (0,843.69) size 769x36
         RenderInline {A} at (0,0) size 88x19 [color=#0000EE]
-          RenderImage {IMG} at (0,0) size 88x31
+          RenderBlock {IMG} at (0,0) size 88x31
       RenderBlock {P} at (0,895.69) size 769x20
         RenderText {#text} at (0,0) size 57x19
           text run at (0,0) width 57: "(Back to "
@@ -297,3 +297,12 @@ layer at (0,0) size 785x989
         RenderInline {A} at (0,0) size 151x19 [color=#0000EE]
           RenderText {#text} at (38,0) size 151x19
             text run at (38,0) width 151: "dbaron@fas.harvard.edu"
+layer at (8,865) size 88x31 clip at (9,866) size 86x29 scrollHeight 62
+  RenderBlock {DIV} at (0,0) size 88x31 [border: (1px solid #C0C0C0)]
+    RenderImage {IMG} at (2,2) size 16x16
+layer at (26,867) size 68x60 backgroundClip at (26,867) size 68x28 clip at (26,867) size 68x28 outlineClip at (9,866) size 86x29
+  RenderBlock {DIV} at (18,2) size 68x60
+    RenderText {#text} at (0,0) size 43x59
+      text run at (0,0) width 32: "Valid"
+      text run at (0,20) width 43: "HTML"
+      text run at (0,40) width 25: "4.0!"