diff --git a/components/layout_2020/flow/mod.rs b/components/layout_2020/flow/mod.rs
index 1aee5748c60..da8f5c9f16a 100644
--- a/components/layout_2020/flow/mod.rs
+++ b/components/layout_2020/flow/mod.rs
@@ -1012,7 +1012,7 @@ fn layout_in_flow_non_replaced_block_level_same_formatting_context(
let end_margin_can_collapse_with_children = collapsed_through ||
(pbm.padding.block_end == Au::zero() &&
pbm.border.block_end == Au::zero() &&
- computed_block_size.is_auto());
+ !containing_block_for_children.size.block.is_definite());
if end_margin_can_collapse_with_children {
block_margins_collapsed_with_children
.end
diff --git a/tests/wpt/meta/MANIFEST.json b/tests/wpt/meta/MANIFEST.json
index c6ce4dfd85b..437f621c427 100644
--- a/tests/wpt/meta/MANIFEST.json
+++ b/tests/wpt/meta/MANIFEST.json
@@ -248765,6 +248765,71 @@
{}
]
],
+ "margin-collapse-with-indefinite-block-size-001.html": [
+ "47d2a0a043d32ba6220c1dd6a26859908a19bf6e",
+ [
+ null,
+ [
+ [
+ "/css/reference/ref-filled-green-100px-square.xht",
+ "=="
+ ]
+ ],
+ {}
+ ]
+ ],
+ "margin-collapse-with-indefinite-block-size-002.html": [
+ "eb401c92b5e9475129f39be9980f47f42cb2bfa0",
+ [
+ null,
+ [
+ [
+ "/css/reference/ref-filled-green-100px-square.xht",
+ "=="
+ ]
+ ],
+ {}
+ ]
+ ],
+ "margin-collapse-with-indefinite-block-size-003.html": [
+ "51f16f68e6998de4956a5f0615b49406fcacb02e",
+ [
+ null,
+ [
+ [
+ "/css/reference/ref-filled-green-100px-square.xht",
+ "=="
+ ]
+ ],
+ {}
+ ]
+ ],
+ "margin-collapse-with-indefinite-block-size-004.html": [
+ "c393560d6c6ce799943db09468ba8e5cb0e90a5c",
+ [
+ null,
+ [
+ [
+ "/css/reference/ref-filled-green-100px-square.xht",
+ "=="
+ ]
+ ],
+ {}
+ ]
+ ],
+ "margin-collapse-with-indefinite-block-size-005.html": [
+ "27814994aaac7ead76f67d051e8744323e8f4247",
+ [
+ null,
+ [
+ [
+ "/css/reference/ref-filled-green-100px-square.xht",
+ "=="
+ ]
+ ],
+ {}
+ ]
+ ],
"max-content-input-001.html": [
"aabbea015748f9276718e2ba9728efc375c8910b",
[
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-001.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-001.html
new file mode 100644
index 00000000000..47d2a0a043d
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-001.html
@@ -0,0 +1,23 @@
+
+
+
+
+
+
+
+
+
+Test passes if there is a filled green square and no red.
+
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-002.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-002.html
new file mode 100644
index 00000000000..eb401c92b5e
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-002.html
@@ -0,0 +1,23 @@
+
+
+
+
+
+
+
+
+
+Test passes if there is a filled green square and no red.
+
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-003.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-003.html
new file mode 100644
index 00000000000..51f16f68e69
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-003.html
@@ -0,0 +1,23 @@
+
+
+
+
+
+
+
+
+
+Test passes if there is a filled green square and no red.
+
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-004.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-004.html
new file mode 100644
index 00000000000..c393560d6c6
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-004.html
@@ -0,0 +1,23 @@
+
+
+
+
+
+
+
+
+
+Test passes if there is a filled green square and no red.
+
diff --git a/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-005.html b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-005.html
new file mode 100644
index 00000000000..27814994aaa
--- /dev/null
+++ b/tests/wpt/tests/css/css-sizing/margin-collapse-with-indefinite-block-size-005.html
@@ -0,0 +1,23 @@
+
+
+
+
+
+
+
+
+
+Test passes if there is a filled green square and no red.
+