diff --git a/components/layout/block.rs b/components/layout/block.rs
index 4f69a9487d8..fa91b847b20 100644
--- a/components/layout/block.rs
+++ b/components/layout/block.rs
@@ -1751,7 +1751,7 @@ impl Flow for BlockFlow {
self.base.position.size.block = self.fragment.border_box.size.block;
}
None
- } else if self.is_root() || self.base.flags.is_float() || self.is_inline_block() {
+ } else if self.is_root() || self.formatting_context_type() != FormattingContextType::None {
// Root element margins should never be collapsed according to CSS ยง 8.3.1.
debug!("assign_block_size: assigning block_size for root flow {:?}",
flow::base(self).debug_id());
diff --git a/tests/wpt/metadata-css/css21_dev/html4/margin-collapse-009.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/margin-collapse-009.htm.ini
deleted file mode 100644
index 6627a225d8a..00000000000
--- a/tests/wpt/metadata-css/css21_dev/html4/margin-collapse-009.htm.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[margin-collapse-009.htm]
- type: reftest
- expected: FAIL
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index 69c05eaecdc..121842c43ff 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -579,6 +579,18 @@
"url": "/_mozilla/css/block_formatting_context_float_placement_a.html"
}
],
+ "css/block_formatting_context_margin_collapse_a.html": [
+ {
+ "path": "css/block_formatting_context_margin_collapse_a.html",
+ "references": [
+ [
+ "/_mozilla/css/block_formatting_context_margin_collapse_ref.html",
+ "=="
+ ]
+ ],
+ "url": "/_mozilla/css/block_formatting_context_margin_collapse_a.html"
+ }
+ ],
"css/block_formatting_context_max_width_a.html": [
{
"path": "css/block_formatting_context_max_width_a.html",
@@ -7193,6 +7205,18 @@
"url": "/_mozilla/css/block_formatting_context_float_placement_a.html"
}
],
+ "css/block_formatting_context_margin_collapse_a.html": [
+ {
+ "path": "css/block_formatting_context_margin_collapse_a.html",
+ "references": [
+ [
+ "/_mozilla/css/block_formatting_context_margin_collapse_ref.html",
+ "=="
+ ]
+ ],
+ "url": "/_mozilla/css/block_formatting_context_margin_collapse_a.html"
+ }
+ ],
"css/block_formatting_context_max_width_a.html": [
{
"path": "css/block_formatting_context_max_width_a.html",
diff --git a/tests/wpt/mozilla/tests/css/block_formatting_context_margin_collapse_a.html b/tests/wpt/mozilla/tests/css/block_formatting_context_margin_collapse_a.html
new file mode 100644
index 00000000000..787505d08c2
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/block_formatting_context_margin_collapse_a.html
@@ -0,0 +1,28 @@
+
+
+
+
+
+
+
diff --git a/tests/wpt/mozilla/tests/css/block_formatting_context_margin_collapse_ref.html b/tests/wpt/mozilla/tests/css/block_formatting_context_margin_collapse_ref.html
new file mode 100644
index 00000000000..3dfb8717acf
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/block_formatting_context_margin_collapse_ref.html
@@ -0,0 +1,31 @@
+
+
+
+
+
+
+