3 files changed, 52 insertions, 0 deletions
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size-quirky-body-iframe.html b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size-quirky-body-iframe.html
new file mode 100644
index 0000000000..da770e6caf
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size-quirky-body-iframe.html
@@ -0,0 +1,21 @@
+<style>
+body {
+  margin: 0;
+  --expected-block-size: 200;
+}
+
+.child {
+  background: green;
+}
+
+@supports (display: layout(test)) {
+  body {
+    display: layout(test);
+  }
+}
+</style>
+
+<!-- In Quirks mode, we should stretch to 100% of the inital containing block. -->
+<body>
+<div class="child"></div>
+</body>
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size.js b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size.js
new file mode 100644
index 0000000000..25d73ef615
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-block-size.js
@@ -0,0 +1,22 @@
+registerLayout('test', class {
+  static get inputProperties() {
+    return ['--expected-block-size'];
+  }
+
+  async intrinsicSizes() {}
+  async layout([child], edges, constraints, styleMap) {
+    let childFixedInlineSize = 0;
+    let childFixedBlockSize = 0;
+    if (constraints.fixedBlockSize === JSON.parse(styleMap.get('--expected-block-size'))) {
+      childFixedInlineSize = 100;
+      childFixedBlockSize = 100;
+    }
+
+    const childFragments = [await child.layoutNextFragment({
+      fixedInlineSize: childFixedInlineSize,
+      fixedBlockSize: childFixedBlockSize,
+    })];
+
+    return {childFragments};
+  }
+});
diff --git a/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-inline-size.js b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-inline-size.js
new file mode 100644
index 0000000000..3636f36654
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints/support/constraints-fixed-inline-size.js
@@ -0,0 +1,9 @@
+registerLayout('test', class {
+  async intrinsicSizes() {}
+  async layout(children, edges, constraints, styleMap) {
+    if (constraints.fixedInlineSize !== 100)
+      return {autoBlockSize: 0};
+
+    return {autoBlockSize: 100};
+  }
+});