Auto merge of #25763 - mrobinson:stacking-contexts-v1, r=SimonSapin

Add initial support for stacking contexts to layout_2020

These changes add initial support for stacking contexts, enabling some parts of z-index to work properly.

---
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- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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tests/wpt/include-layout-2020.ini

@@ -13,6 +13,10 @@ skip: true
       skip: false
     [box-display]
       skip: false
+    [zindex]
+      skip: false
+    [zorder]
+      skip: false
   [css-backgrounds]
     skip: false
   [cssom-view]

tests/wpt/metadata-layout-2020/css/CSS2/zindex/stack-floats-001.xht.ini

@@ -0,0 +1,2 @@
+[stack-floats-001.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/stack-floats-002.xht.ini

@@ -0,0 +1,2 @@
+[stack-floats-002.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/stack-floats-003.xht.ini

@@ -0,0 +1,2 @@
+[stack-floats-003.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/stack-floats-004.xht.ini

@@ -0,0 +1,2 @@
+[stack-floats-004.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-015.xht.ini

@@ -0,0 +1,2 @@
+[z-index-015.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-016.xht.ini

@@ -0,0 +1,2 @@
+[z-index-016.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-017.xht.ini

@@ -0,0 +1,2 @@
+[z-index-017.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-018.xht.ini

@@ -0,0 +1,2 @@
+[z-index-018.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-019.xht.ini

@@ -0,0 +1,2 @@
+[z-index-019.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-abspos-001.xht.ini

@@ -0,0 +1,2 @@
+[z-index-abspos-001.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-abspos-002.xht.ini

@@ -0,0 +1,2 @@
+[z-index-abspos-002.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-abspos-003.xht.ini

@@ -0,0 +1,2 @@
+[z-index-abspos-003.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-abspos-004.xht.ini

@@ -0,0 +1,2 @@
+[z-index-abspos-004.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-abspos-005.xht.ini

@@ -0,0 +1,2 @@
+[z-index-abspos-005.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zindex/z-index-abspos-007.xht.ini

@@ -0,0 +1,2 @@
+[z-index-abspos-007.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/CSS2/zorder/z-index-020.xht.ini

@@ -0,0 +1,2 @@
+[z-index-020.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-backgrounds/css3-background-origin-border-box.html.ini

@@ -1,2 +0,0 @@
-[css3-background-origin-border-box.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/css-position-3/position-sticky-stacking-context.html.ini

@@ -0,0 +1,2 @@
+[position-sticky-stacking-context.html]
+  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/opacity_stacking_context_a.html.ini

@@ -0,0 +1,2 @@
+[opacity_stacking_context_a.html]
+  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/position_relative_painting_order_a.html.ini

@@ -1,2 +0,0 @@
-[position_relative_painting_order_a.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/position_relative_stacking_context_a.html.ini

@@ -1,2 +0,0 @@
-[position_relative_stacking_context_a.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/transform_stacking_context_a.html.ini

@@ -0,0 +1,2 @@
+[transform_stacking_context_a.html]
+  expected: FAIL