--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -120790,16 +120790,52 @@
       [
        "/css/css-layout-api/box-tree-unregistered-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
+   "css/css-layout-api/constraints-data-function-failure.https.html": [
+    [
+     "/css/css-layout-api/constraints-data-function-failure.https.html",
+     [
+      [
+       "/css/css-layout-api/green-square-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
+   "css/css-layout-api/constraints-data-sab-failure.https.html": [
+    [
+     "/css/css-layout-api/constraints-data-sab-failure.https.html",
+     [
+      [
+       "/css/css-layout-api/green-square-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
+   "css/css-layout-api/constraints-data.https.html": [
+    [
+     "/css/css-layout-api/constraints-data.https.html",
+     [
+      [
+       "/css/css-layout-api/green-square-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/css-layout-api/constraints-fixed-inline-size-absolute-left-right.https.html": [
     [
      "/css/css-layout-api/constraints-fixed-inline-size-absolute-left-right.https.html",
      [
       [
        "/css/css-layout-api/constraints-fixed-inline-size-ref.html",
        "=="
       ]
@@ -121018,190 +121054,238 @@
       [
        "/css/css-layout-api/fallback-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
+   "css/css-layout-api/fragment-data-function-failure.https.html": [
+    [
+     "/css/css-layout-api/fragment-data-function-failure.https.html",
+     [
+      [
+       "/css/css-layout-api/green-square-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
+   "css/css-layout-api/fragment-data-immutable.https.html": [
+    [
+     "/css/css-layout-api/fragment-data-immutable.https.html",
+     [
+      [
+       "/css/css-layout-api/green-square-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
+   "css/css-layout-api/fragment-data-sab-failure.https.html": [
+    [
+     "/css/css-layout-api/fragment-data-sab-failure.https.html",
+     [
+      [
+       "/css/css-layout-api/green-square-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
+   "css/css-layout-api/fragment-data.https.html": [
+    [
+     "/css/css-layout-api/fragment-data.https.html",
+     [
+      [
+       "/css/css-layout-api/green-square-ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/css-layout-api/layout-child-absolute.https.html": [
     [
      "/css/css-layout-api/layout-child-absolute.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/layout-child-before-after.https.html": [
     [
      "/css/css-layout-api/layout-child-before-after.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/layout-child-fixed.https.html": [
     [
      "/css/css-layout-api/layout-child-fixed.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/layout-child-float.https.html": [
     [
      "/css/css-layout-api/layout-child-float.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/layout-child-inflow.https.html": [
     [
      "/css/css-layout-api/layout-child-inflow.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/layout-child-inlines.https.html": [
     [
      "/css/css-layout-api/layout-child-inlines.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/layout-child-text.https.html": [
     [
      "/css/css-layout-api/layout-child-text.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/perform-child-layout-available-inline-size-htb-htb.https.html": [
     [
      "/css/css-layout-api/perform-child-layout-available-inline-size-htb-htb.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/perform-child-layout-available-inline-size-htb-vrl.https.html": [
     [
      "/css/css-layout-api/perform-child-layout-available-inline-size-htb-vrl.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/perform-child-layout-available-inline-size-vrl-htb.https.html": [
     [
      "/css/css-layout-api/perform-child-layout-available-inline-size-vrl-htb.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/perform-child-layout-available-inline-size-vrl-vrl.https.html": [
     [
      "/css/css-layout-api/perform-child-layout-available-inline-size-vrl-vrl.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/perform-child-layout-fixed-block-size-vrl.https.html": [
     [
      "/css/css-layout-api/perform-child-layout-fixed-block-size-vrl.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/perform-child-layout-fixed-block-size.https.html": [
     [
      "/css/css-layout-api/perform-child-layout-fixed-block-size.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/perform-child-layout-fixed-inline-size-vrl.https.html": [
     [
      "/css/css-layout-api/perform-child-layout-fixed-inline-size-vrl.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/perform-child-layout-fixed-inline-size.https.html": [
     [
      "/css/css-layout-api/perform-child-layout-fixed-inline-size.https.html",
      [
       [
-       "/css/css-layout-api/layout-child-ref.html",
+       "/css/css-layout-api/green-square-ref.html",
        "=="
       ]
      ],
      {}
     ]
    ],
    "css/css-layout-api/position-fragment-htb-ltr.https.html": [
     [
@@ -250819,17 +250903,17 @@
      {}
     ]
    ],
    "css/css-layout-api/fallback-ref.html": [
     [
      {}
     ]
    ],
-   "css/css-layout-api/layout-child-ref.html": [
+   "css/css-layout-api/green-square-ref.html": [
     [
      {}
     ]
    ],
    "css/css-layout-api/position-fragment-ref.html": [
     [
      {}
     ]
@@ -516716,16 +516800,28 @@
   "css/css-layout-api/box-tree-unregistered.https.html": [
    "c2058c848fe71230148b2531585fbddd678e6926",
    "reftest"
   ],
   "css/css-layout-api/computed-style-layout-function.https.html": [
    "a999a2a7ad8981879cdc984bf602939a23a60ea6",
    "testharness"
   ],
+  "css/css-layout-api/constraints-data-function-failure.https.html": [
+   "5158c7620d4e5aa9d88455fbbd88277572cdcb63",
+   "reftest"
+  ],
+  "css/css-layout-api/constraints-data-sab-failure.https.html": [
+   "ce21d5ca8c3059f7f1a6981bda3ff37802cea27d",
+   "reftest"
+  ],
+  "css/css-layout-api/constraints-data.https.html": [
+   "371958d97ba951172a283a653f2064def1327a32",
+   "reftest"
+  ],
   "css/css-layout-api/constraints-fixed-inline-size-absolute-left-right.https.html": [
    "15f8dde88dd066b5525a6b8990206ac567f48b2a",
    "reftest"
   ],
   "css/css-layout-api/constraints-fixed-inline-size-absolute-top-bottom-vrl.https.html": [
    "9cb3a2b8671ef4f9926a39431d193875a5a1738a",
    "reftest"
   ],
@@ -516804,82 +516900,98 @@
   "css/css-layout-api/fallback-layout-return.https.html": [
    "159662be847f8657d33a0607cb0e6561d57ef4e1",
    "reftest"
   ],
   "css/css-layout-api/fallback-ref.html": [
    "464a929254bba9991b03e2d0d161c0274c7c4b59",
    "support"
   ],
+  "css/css-layout-api/fragment-data-function-failure.https.html": [
+   "3237c10bd95c4d0b25b8fe2db8768b5f7f50f18b",
+   "reftest"
+  ],
+  "css/css-layout-api/fragment-data-immutable.https.html": [
+   "d0321eef8d3412607090ac77ef9b160ff6b2a3cd",
+   "reftest"
+  ],
+  "css/css-layout-api/fragment-data-sab-failure.https.html": [
+   "bc91309fddc5146b0258a4a9e362625c1ef1e5cb",
+   "reftest"
+  ],
+  "css/css-layout-api/fragment-data.https.html": [
+   "525c663bd7e9844af8f42429d936558b95a66188",
+   "reftest"
+  ],
+  "css/css-layout-api/green-square-ref.html": [
+   "925277c0e5cc163b9f39d878b9b39d7f822dda51",
+   "support"
+  ],
   "css/css-layout-api/inline-style-layout-function.https.html": [
    "f421a9c0fb4c930922281a88eeaa45ae69adab51",
    "testharness"
   ],
   "css/css-layout-api/layout-child-absolute.https.html": [
-   "39c26ad4eb1c455e911ad1d963494096f8560f9a",
+   "42a689b586a297b9f40f18eee0806d587da00351",
    "reftest"
   ],
   "css/css-layout-api/layout-child-before-after.https.html": [
-   "77408947564e138eff1f85fa71be05a0303bc9dc",
+   "6836c7c8b81adda98e8de4041e0b505d1e0dbdd7",
    "reftest"
   ],
   "css/css-layout-api/layout-child-fixed.https.html": [
-   "61fafcd052444e4d996748b9c132101b754747e7",
+   "ca84f12d5982ef9f1a6531f41721d76bf2a1c8c6",
    "reftest"
   ],
   "css/css-layout-api/layout-child-float.https.html": [
-   "8840ae8518ade98bf38614ef92df20c19b65a667",
+   "a0e2c43f60ba1376090a32df5e9625faaac23274",
    "reftest"
   ],
   "css/css-layout-api/layout-child-inflow.https.html": [
-   "d1034937a934ac7900ecd59d958de00c769a73cf",
+   "af6fdd340c5e81b02dc99012cc990c0fcc9e68bb",
    "reftest"
   ],
   "css/css-layout-api/layout-child-inlines.https.html": [
-   "048d886c9388151a792bdc12fb8098aa53249cfd",
-   "reftest"
-  ],
-  "css/css-layout-api/layout-child-ref.html": [
-   "1688098230f9fb825bc73134dff15dcf5ff9782d",
-   "support"
+   "fd8144258a9c4ce17e2fb0fc7439060341ad93e7",
+   "reftest"
   ],
   "css/css-layout-api/layout-child-text.https.html": [
-   "567ff67317cae9906e6d159dc232c41464a4e7c6",
+   "82a8fada6d99e40410ab415e2b34af219dac94c0",
    "reftest"
   ],
   "css/css-layout-api/perform-child-layout-available-inline-size-htb-htb.https.html": [
-   "7db4f5c2bebe031f0689be2569ba4ca266535052",
+   "5671810501fc3095f8c72b94a1b4ef03711683c3",
    "reftest"
   ],
   "css/css-layout-api/perform-child-layout-available-inline-size-htb-vrl.https.html": [
-   "f22cc9627b7b2ad797cc722f3d369355ffce4f60",
+   "a7c8ead348cc60e4a75b9f059411a0e5da02f469",
    "reftest"
   ],
   "css/css-layout-api/perform-child-layout-available-inline-size-vrl-htb.https.html": [
-   "0e34d5b8b86aa6551e6e6a31eb086bd84b61c636",
+   "4e983d5db151d48f39a018394f2d23bbecbed347",
    "reftest"
   ],
   "css/css-layout-api/perform-child-layout-available-inline-size-vrl-vrl.https.html": [
-   "d619b57c395710bc2e133e05774c9a06bfd19749",
+   "52edc3ff48579820b9511de7930aa5f72a7b3c59",
    "reftest"
   ],
   "css/css-layout-api/perform-child-layout-fixed-block-size-vrl.https.html": [
-   "907b9b1255516caab731555686e5c6b5517eb3f9",
+   "3fa3ddc4c728eaa1e120c9aa7845dd60881c6348",
    "reftest"
   ],
   "css/css-layout-api/perform-child-layout-fixed-block-size.https.html": [
-   "3560b1f8249f945e1453e3e8e53745ef1941405f",
+   "67ffdaf070d916be1867105239bc477ac2c553ea",
    "reftest"
   ],
   "css/css-layout-api/perform-child-layout-fixed-inline-size-vrl.https.html": [
-   "dfc48415ddbfecad7b382c4efe290b1b8da4ebac",
+   "f341a69a76cfd564117f57d67e4740d73592fbf8",
    "reftest"
   ],
   "css/css-layout-api/perform-child-layout-fixed-inline-size.https.html": [
-   "bae125f32f28d0cfa7db449cfb10437cb4df12a3",
+   "3f1a6187f48a669d5032f9b161926b588a7ed968",
    "reftest"
   ],
   "css/css-layout-api/position-fragment-htb-ltr.https.html": [
    "761ed3e46d6485b1208953c292015432da5f99e0",
    "reftest"
   ],
   "css/css-layout-api/position-fragment-htb-rtl.https.html": [
    "8a2684c25ef3603e498cf33d32dc8a2c4b4a50d5",

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints-data-function-failure.https.html
@@ -0,0 +1,63 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that a function can't be passed to a child layout." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout([child], edges, constraints, styleMap) {
+    let childFragment = null;
+
+    try {
+      childFragment = yield child.layoutNextFragment({
+        data: { fn: function() {} }
+      });
+    } catch(e) {
+      // Success! The structured cloning algorithm should have thrown an error.
+      childFragment = yield child.layoutNextFragment({});
+      return {autoBlockSize: 100, childFragments: [childFragment]};
+    }
+
+    return {autoBlockSize: 0, childFragments: [childFragment]};
+  }
+});
+
+registerLayout('child', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout() {
+    return {autoBlockSize: 0};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints-data-sab-failure.https.html
@@ -0,0 +1,68 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that a SharedArrayBuffer can't be passed to a child layout." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout([child], edges, constraints, styleMap) {
+    let childFragment = null;
+
+    try {
+      // We need SABs to be enabled to properly run this test.
+      if (typeof SharedArrayBuffer !== 'undefined') {
+        childFragment = yield child.layoutNextFragment({
+          data: { sab: new SharedArrayBuffer(4) }
+        });
+      } else {
+        throw Error();
+      }
+    } catch(e) {
+      // Success! The structured cloning algorithm should have thrown an error.
+      childFragment = yield child.layoutNextFragment({});
+      return {autoBlockSize: 100, childFragments: [childFragment]};
+    }
+
+    return {autoBlockSize: 0, childFragments: [childFragment]};
+  }
+});
+
+registerLayout('child', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout() {
+    return {autoBlockSize: 0};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/constraints-data.https.html
@@ -0,0 +1,71 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraints-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that passing data to a child layout works correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+const DATA = {
+  str: 'hello',
+  num: 42,
+  obj: {str2: 'world'},
+};
+
+registerLayout('parent', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout([child], edges, constraints, styleMap) {
+
+    const childFragment = yield child.layoutNextFragment({data: DATA});
+
+    // If the child's block-size is 100 the structured cloning worked.
+    if (childFragment.blockSize === 100) {
+      return {autoBlockSize: 100, childFragments: [childFragment]};
+    }
+
+    return {autoBlockSize: 0, childFragments: [childFragment]};
+  }
+});
+
+registerLayout('child', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout(children, edges, constraints, styleMap) {
+    // Use JSON.stringify to make sure the structured cloning worked.
+    if (constraints.data !== DATA &&
+        JSON.stringify(constraints.data) === JSON.stringify(DATA)) {
+      return {autoBlockSize: 100};
+    }
+
+    return {autoBlockSize: 0};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fragment-data-function-failure.https.html
@@ -0,0 +1,44 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutfragment-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that passing something that can't be serialized to the parent triggers a fallback to block layout." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+.child {
+  height: 100px;
+}
+
+@supports (display: layout(fallback-fn)) {
+  .test {
+    display: layout(fallback-fn);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('fallback-fn', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout(children, edges, constraints, styleMap) {
+    const childFragments = yield children.map(child => child.layoutNextFragment());
+    return {autoBlockSize: 0, childFragments, data: {fn: function() {}}};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fragment-data-immutable.https.html
@@ -0,0 +1,71 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutfragment-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that LayoutFragment#data is immutable between child layout passes." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('parent', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout([child], edges, constraints, styleMap) {
+
+    const childFragment10 = yield child.layoutNextFragment({fixedInlineSize: 10});
+
+    // First layout data should be "10".
+    if (childFragment10.data.size !== 10) {
+      return {autoBlockSize: 0, childFragments: [childFragment10]};
+    }
+
+    const childFragment20 = yield child.layoutNextFragment({fixedInlineSize: 20});
+
+    // Second layout data should be "20".
+    if (childFragment20.data.size !== 20) {
+      return {autoBlockSize: 0, childFragments: [childFragment10]};
+    }
+
+    // First layout data should still be "10".
+    if (childFragment10.data.size !== 10) {
+      return {autoBlockSize: 0, childFragments: [childFragment10]};
+    }
+
+    return {autoBlockSize: 100, childFragments: [childFragment20]};
+  }
+});
+
+registerLayout('child', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout(children, edges, constraints, styleMap) {
+    return {autoBlockSize: 10, data: {size: constraints.fixedInlineSize}};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fragment-data-sab-failure.https.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutfragment-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that a SharedArrayBuffer can't be passed to the parent layout." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+.child {
+  height: 100px;
+}
+
+@supports (display: layout(fallback-sab)) {
+  .test {
+    display: layout(fallback-sab);
+    background: green;
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+registerLayout('fallback-sab', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout(children, edges, constraints, styleMap) {
+    const childFragments = yield children.map(child => child.layoutNextFragment());
+
+    if (typeof SharedArrayBuffer !== 'undefined') {
+      return {autoBlockSize: 0, childFragments, data: {sab: new SharedArrayBuffer(4) }};
+    } else {
+      throw Error();
+    }
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/fragment-data.https.html
@@ -0,0 +1,66 @@
+<!DOCTYPE html>
+<html class=reftest-wait>
+<link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutfragment-data">
+<link rel="match" href="green-square-ref.html">
+<meta name="assert" content="This test checks that passing data to a parent layout works correctly." />
+<style>
+.test {
+  background: red;
+  width: 100px;
+}
+
+@supports (display: layout(parent)) {
+  .test {
+    display: layout(parent);
+    background: green;
+  }
+
+  .child {
+    display: layout(child);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<script src="/common/worklet-reftest.js"></script>
+
+<div class="test">
+  <div class="child"></div>
+</div>
+
+<script id="code" type="text/worklet">
+const DATA = {
+  str: 'hello',
+  num: 42,
+  obj: {str2: 'world'},
+};
+
+registerLayout('parent', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout([child], edges, constraints, styleMap) {
+
+    const childFragment = yield child.layoutNextFragment();
+
+    // Use JSON.stringify to make sure the structured cloning worked.
+    if (childFragment.data !== DATA &&
+        JSON.stringify(childFragment.data) === JSON.stringify(DATA)) {
+      return {autoBlockSize: 100, childFragments: [childFragment]};
+    }
+
+    return {autoBlockSize: 0, childFragments: [childFragment]};
+  }
+});
+
+registerLayout('child', class {
+  *intrinsicSizes() {}
+  *layout() {}
+  *layout(children, edges, constraints, styleMap) {
+    return {autoBlockSize: 10, data: DATA};
+  }
+});
+</script>
+
+<script>
+importWorkletAndTerminateTestAfterAsyncPaint(CSS.layoutWorklet, document.getElementById('code').textContent);
+</script>
+</html>

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/css/css-layout-api/green-square-ref.html
@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<style>
+.result {
+  background: green;
+  height: 100px;
+  width: 100px;
+}
+</style>
+<div class="result"></div>

--- a/testing/web-platform/tests/css/css-layout-api/layout-child-absolute.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child-absolute.https.html
@@ -1,20 +1,19 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that absolute children don't appear in the children array." />
 
 <style>
 .test {
   --child-expected: ["2"];
 
   background: red;
-  margin: 10px;
   width: 100px;
 }
 
 .absolute {
   position: absolute;
   visibility: hidden;
   --child: 1;
 }

--- a/testing/web-platform/tests/css/css-layout-api/layout-child-before-after.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child-before-after.https.html
@@ -1,20 +1,19 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that boxes created by ::before/::after appear as children." />
 
 <style>
 .test {
   --child-expected: ["1", "2", "3"];
 
   background: red;
-  margin: 10px;
   width: 100px;
 }
 
 .test::before {
   visibility: hidden;
   content: 'before';
   --child: 1;
 }

--- a/testing/web-platform/tests/css/css-layout-api/layout-child-fixed.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child-fixed.https.html
@@ -1,20 +1,19 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that fixed children don't appear in the children array." />
 
 <style>
 .test {
   --child-expected: ["2"];
 
   background: red;
-  margin: 10px;
   width: 100px;
 }
 
 .fixed {
   position: fixed;
   visibility: hidden;
   --child: 1;
 }

--- a/testing/web-platform/tests/css/css-layout-api/layout-child-float.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child-float.https.html
@@ -1,20 +1,19 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that float children appear in the children array." />
 
 <style>
 .test {
   --child-expected: ["1", "2"];
 
   background: red;
-  margin: 10px;
   width: 100px;
 }
 
 .float {
   float: right;
   visibility: hidden;
   --child: 1;
 }

--- a/testing/web-platform/tests/css/css-layout-api/layout-child-inflow.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child-inflow.https.html
@@ -1,20 +1,19 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that regular inflow children appear as children." />
 
 <style>
 .test {
   --child-expected: ["1", "2"];
 
   background: red;
-  margin: 10px;
   width: 100px;
 }
 
 .inflow-1 {
   visibility: hidden;
   --child: 1;
 }
 

--- a/testing/web-platform/tests/css/css-layout-api/layout-child-inlines.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child-inlines.https.html
@@ -1,22 +1,21 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that inline children are correctly blockified or wrapped in anonymous boxes." />
 
 <style>
 /* We have a wrapper in this test to ensure that any text that is positioned
  * slightly outside the "test" box doesn't affect the rendering.
  * This wrapper has a 10px inline padding which does the trick. */
 .wrapper {
   background: green;
   padding: 0 10px;
-  margin: 10px;
   width: 80px;
 }
 
 .test {
   --child-expected: ["1", "default", "3", "4", "5"];
 
   background: red;
   color: green;

deleted file mode 100644
--- a/testing/web-platform/tests/css/css-layout-api/layout-child-ref.html
+++ /dev/null
@@ -1,10 +0,0 @@
-<!DOCTYPE html>
-<style>
-.result {
-  background: green;
-  margin: 10px;
-  height: 100px;
-  width: 100px;
-}
-</style>
-<div class="result"></div>

--- a/testing/web-platform/tests/css/css-layout-api/layout-child-text.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/layout-child-text.https.html
@@ -1,22 +1,21 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#layout-children">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that text children are correctly blockified." />
 
 <style>
 /* We have a wrapper in this test to ensure that any text that is positioned
  * slightly outside the "test" box doesn't affect the rendering.
  * This wrapper has a 10px inline padding which does the trick. */
 .wrapper {
   background: green;
   padding: 0 10px;
-  margin: 10px;
   width: 80px;
 }
 
 .test {
   --child-expected: ["default", "2", "default"];
 
   background: red;
   color: green;

--- a/testing/web-platform/tests/css/css-layout-api/perform-child-layout-available-inline-size-htb-htb.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/perform-child-layout-available-inline-size-htb-htb.https.html
@@ -1,19 +1,18 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that setting the available inline size of children works as expected." />
 
 <style>
 .test {
   writing-mode: horizontal-tb;
   background: red;
-  margin: 10px;
   width: 100px;
 }
 
 .child {
   writing-mode: horizontal-tb;
   visibility: hidden;
   line-height: 0;
 

--- a/testing/web-platform/tests/css/css-layout-api/perform-child-layout-available-inline-size-htb-vrl.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/perform-child-layout-available-inline-size-htb-vrl.https.html
@@ -1,19 +1,18 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that setting the available inline size of children works as expected." />
 
 <style>
 .test {
   writing-mode: horizontal-tb;
   background: red;
-  margin: 10px;
   width: 100px;
 }
 
 .child {
   writing-mode: vertical-rl;
   visibility: hidden;
   line-height: 0;
 

--- a/testing/web-platform/tests/css/css-layout-api/perform-child-layout-available-inline-size-vrl-htb.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/perform-child-layout-available-inline-size-vrl-htb.https.html
@@ -1,19 +1,18 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that setting the available inline size of children works as expected." />
 
 <style>
 .test {
   writing-mode: vertical-rl;
   background: red;
-  margin: 10px;
   height: 100px;
 }
 
 .child {
   writing-mode: horizontal-tb;
   visibility: hidden;
   line-height: 0;
 

--- a/testing/web-platform/tests/css/css-layout-api/perform-child-layout-available-inline-size-vrl-vrl.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/perform-child-layout-available-inline-size-vrl-vrl.https.html
@@ -1,19 +1,18 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-availableinlinesize">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that setting the available inline size of children works as expected." />
 
 <style>
 .test {
   writing-mode: vertical-rl;
   background: red;
-  margin: 10px;
   height: 100px;
 }
 
 .child {
   writing-mode: vertical-rl;
   visibility: hidden;
   line-height: 0;
 

--- a/testing/web-platform/tests/css/css-layout-api/perform-child-layout-fixed-block-size-vrl.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/perform-child-layout-fixed-block-size-vrl.https.html
@@ -1,19 +1,18 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-fixedinlinesize">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that fixing the block size of children works as expected." />
 
 <style>
 .test {
   writing-mode: vertical-rl;
   background: red;
-  margin: 10px;
   height: 100px;
 }
 
 .htb {
   writing-mode: horizontal-tb;
   visibility: hidden;
   width: 3px;
   height: 2px;

--- a/testing/web-platform/tests/css/css-layout-api/perform-child-layout-fixed-block-size.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/perform-child-layout-fixed-block-size.https.html
@@ -1,18 +1,17 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-fixedblocksize">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that fixing the block size of children works as expected." />
 
 <style>
 .test {
   background: red;
-  margin: 10px;
   width: 100px;
 }
 
 .htb {
   writing-mode: horizontal-tb;
   visibility: hidden;
   width: 3px;
   height: 2px;

--- a/testing/web-platform/tests/css/css-layout-api/perform-child-layout-fixed-inline-size-vrl.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/perform-child-layout-fixed-inline-size-vrl.https.html
@@ -1,19 +1,18 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-fixedinlinesize">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that fixing the inline size of children works as expected." />
 
 <style>
 .test {
   writing-mode: vertical-rl;
   background: red;
-  margin: 10px;
   height: 100px;
 }
 
 .htb {
   writing-mode: horizontal-tb;
   visibility: hidden;
   width: 3px;
   height: 2px;

--- a/testing/web-platform/tests/css/css-layout-api/perform-child-layout-fixed-inline-size.https.html
+++ b/testing/web-platform/tests/css/css-layout-api/perform-child-layout-fixed-inline-size.https.html
@@ -1,18 +1,17 @@
 <!DOCTYPE html>
 <html class=reftest-wait>
 <link rel="help" href="https://drafts.css-houdini.org/css-layout-api/#dom-layoutconstraintsoptions-fixedinlinesize">
-<link rel="match" href="layout-child-ref.html">
+<link rel="match" href="green-square-ref.html">
 <meta name="assert" content="This test checks that fixing the inline size of children works as expected." />
 
 <style>
 .test {
   background: red;
-  margin: 10px;
   width: 100px;
 }
 
 .htb {
   writing-mode: horizontal-tb;
   visibility: hidden;
   width: 3px;
   height: 2px;