diff --git a/components/style/properties.mako.rs b/components/style/properties.mako.rs
index 3f66a733358..93b95a60e80 100644
--- a/components/style/properties.mako.rs
+++ b/components/style/properties.mako.rs
@@ -5732,6 +5732,12 @@ pub fn cascade(viewport_size: Size2D<Au>,
         box_.display = box_.display.to_computed_value(&context);
     }
 
+    // The initial value of outline width may be changed at computed value time.
+    if !context.outline_style_present {
+        let outline = unsafe { style_outline.make_unique() };
+        outline.outline_width = Au(0);
+    }
+
     if is_root_element {
         context.root_font_size = context.font_size;
     }
@@ -5779,6 +5785,11 @@ pub fn cascade_anonymous(parent_style: &ComputedValues) -> ComputedValues {
             border.border_${side}_width = Au(0);
         % endfor
     }
+    {
+        // Initial value of outline-style is always none for anonymous box.
+        let outline = unsafe { result.outline.make_unique() };
+        outline.outline_width = Au(0);
+    }
     // None of the teaks on 'display' apply here.
     result
 }
diff --git a/tests/wpt/metadata-css/css21_dev/html4/abspos-containing-block-001.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/abspos-containing-block-001.htm.ini
deleted file mode 100644
index 6a628438496..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/abspos-containing-block-001.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[abspos-containing-block-001.htm]
-  type: reftest
-  expected: FAIL
diff --git a/tests/wpt/metadata-css/css21_dev/html4/abspos-containing-block-003.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/abspos-containing-block-003.htm.ini
deleted file mode 100644
index 53001ffedae..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/abspos-containing-block-003.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[abspos-containing-block-003.htm]
-  type: reftest
-  expected: FAIL