From 1f3329bb4ed7f579252983aa98d963c120438afc Mon Sep 17 00:00:00 2001
From: Yoav Weiss <yoavweiss@chromium.org>
Date: Wed, 6 Nov 2024 05:27:13 -0800
Subject: [PATCH] Multiple import maps

Import maps currently have to load before any ES module and there can
only be a single import map per document. That makes them fragile and
potentially slow to use in real-life scenarios: Any module that loads
before them breaks the entire app, and in apps with many modules the
become a large blocking resource, as the entire map for all possible
modules needs to load first.

This implements https://github.com/whatwg/html/pull/10528 to solve that.

Change-Id: I54e1b9cdfe989d61c85d73a5fd384f860273ad9a
Bug: 358379381
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5776262
Commit-Queue: Yoav Weiss (@Shopify) <yoavweiss@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378943}
---
 import-maps/acquiring/dynamic-import.html     |  9 +--
 .../acquiring/modulepreload-link-header.html  |  8 +-
 import-maps/acquiring/modulepreload.html      |  8 +-
 import-maps/acquiring/script-tag-inline.html  | 13 +--
 import-maps/acquiring/script-tag.html         |  8 +-
 .../already-resolved-dropped.html             | 26 ++++++
 import-maps/multiple-import-maps/basic.html   |  6 +-
 .../conflict-first-persists.html              | 49 +++++++++++
 ...ion-consistency-in-module-tree-inline.html | 45 +++++++++++
 ...resolution-consistency-in-module-tree.html | 33 ++++++++
 .../url-resolution-conflict.html              | 32 ++++++++
 .../multiple-import-maps/with-errors.html     |  8 +-
 import-maps/not-overridden/dynamic.html       | 40 +++++++++
 .../external-script-bare-descendent.html      | 52 ++++++++++++
 ...external-script-descendent-with-scope.html | 81 +++++++++++++++++++
 .../external-script-descendent.html           | 40 +++++++++
 .../not-overridden/failed-resolution.html     | 60 ++++++++++++++
 import-maps/not-overridden/integrity.html     | 38 +++++++++
 .../modulepreload-header.html.headers         |  2 +
 import-maps/not-overridden/prefix.html        | 40 +++++++++
 .../not-overridden/script-descendent.html     | 33 ++++++++
 .../url-resolution-conflict.html              | 28 +++++++
 import-maps/resources/importer.sub.js         |  1 +
 import-maps/resources/in-scope-test.js        |  3 +
 import-maps/resources/log.sub.js              |  2 +
 import-maps/resources/test-helper.js          |  8 ++
 resources/out-of-scope-test.js                |  5 ++
 27 files changed, 637 insertions(+), 41 deletions(-)
 create mode 100644 import-maps/multiple-import-maps/already-resolved-dropped.html
 create mode 100644 import-maps/multiple-import-maps/conflict-first-persists.html
 create mode 100644 import-maps/multiple-import-maps/resolution-consistency-in-module-tree-inline.html
 create mode 100644 import-maps/multiple-import-maps/resolution-consistency-in-module-tree.html
 create mode 100644 import-maps/multiple-import-maps/url-resolution-conflict.html
 create mode 100644 import-maps/not-overridden/dynamic.html
 create mode 100644 import-maps/not-overridden/external-script-bare-descendent.html
 create mode 100644 import-maps/not-overridden/external-script-descendent-with-scope.html
 create mode 100644 import-maps/not-overridden/external-script-descendent.html
 create mode 100644 import-maps/not-overridden/failed-resolution.html
 create mode 100644 import-maps/not-overridden/integrity.html
 create mode 100644 import-maps/not-overridden/modulepreload-header.html.headers
 create mode 100644 import-maps/not-overridden/prefix.html
 create mode 100644 import-maps/not-overridden/script-descendent.html
 create mode 100644 import-maps/not-overridden/url-resolution-conflict.html
 create mode 100644 import-maps/resources/importer.sub.js
 create mode 100644 import-maps/resources/in-scope-test.js
 create mode 100644 import-maps/resources/log.sub.js
 create mode 100644 resources/out-of-scope-test.js

diff --git a/import-maps/acquiring/dynamic-import.html b/import-maps/acquiring/dynamic-import.html
index 49d8a701da2e43..2521b2fb7f4dec 100644
--- a/import-maps/acquiring/dynamic-import.html
+++ b/import-maps/acquiring/dynamic-import.html
@@ -3,8 +3,6 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script>
-const t = async_test(
-  'After dynamic imports, import maps should fire error events');
 const log = [];
 // To ensure we are testing that the flag is cleared at the beginning of module
 // script loading unconditionally, not at the end of loading or not at the
@@ -14,7 +12,7 @@
 promise_test(() => import('../resources/empty.js?pipe=trickle(d1)'),
              "A dynamic import succeeds");
 </script>
-<script type="importmap" onload="t.assert_unreached('onload')" onerror="t.done()">
+<script type="importmap">
 {
   "imports": {
     "../resources/log.js?pipe=sub&name=A": "../resources/log.js?pipe=sub&name=B"
@@ -24,7 +22,6 @@
 <script>
 promise_test(() => {
   return import("../resources/log.js?pipe=sub&name=A")
-    .then(() => assert_array_equals(log, ["log:A"]))
-  },
-  'After a dynamic import(), import maps are not effective');
+    .then(() => assert_array_equals(log, ["log:B"]))
+  }, 'After a dynamic import(), import maps work fine');
 </script>
diff --git a/import-maps/acquiring/modulepreload-link-header.html b/import-maps/acquiring/modulepreload-link-header.html
index dde8cabb9323f0..5d3eaabd8f5217 100644
--- a/import-maps/acquiring/modulepreload-link-header.html
+++ b/import-maps/acquiring/modulepreload-link-header.html
@@ -3,11 +3,9 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script>
-const t = async_test(
-  'With modulepreload link header, import maps should fire error events');
 const log = [];
 </script>
-<script type="importmap" onerror="t.done()">
+<script type="importmap">
 {
   "imports": {
     "../resources/log.js?pipe=sub&name=A": "../resources/log.js?pipe=sub&name=B"
@@ -17,7 +15,7 @@
 <script>
 promise_test(() => {
   return import("../resources/log.js?pipe=sub&name=A")
-    .then(() => assert_array_equals(log, ["log:A"]))
+    .then(() => assert_array_equals(log, ["log:B"]))
   },
-  'With modulepreload link header, import maps are not effective');
+  'With modulepreload link header, import maps work fine');
 </script>
diff --git a/import-maps/acquiring/modulepreload.html b/import-maps/acquiring/modulepreload.html
index 68b66a8ae330e4..0571c7dcbe4c4f 100644
--- a/import-maps/acquiring/modulepreload.html
+++ b/import-maps/acquiring/modulepreload.html
@@ -3,12 +3,10 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script>
-const t = async_test(
-  'After <link rel=modulepreload> import maps should fire error events');
 const log = [];
 </script>
 <link rel="modulepreload" href="../resources/empty.js?pipe=trickle(d1)"></link>
-<script type="importmap" onerror="t.done()">
+<script type="importmap">
 {
   "imports": {
     "../resources/log.js?pipe=sub&name=A": "../resources/log.js?pipe=sub&name=B"
@@ -18,7 +16,7 @@
 <script>
 promise_test(() => {
   return import("../resources/log.js?pipe=sub&name=A")
-    .then(() => assert_array_equals(log, ["log:A"]))
+    .then(() => assert_array_equals(log, ["log:B"]))
   },
-  'After <link rel=modulepreload> import maps are not effective');
+  'After <link rel=modulepreload> import maps should work fine');
 </script>
diff --git a/import-maps/acquiring/script-tag-inline.html b/import-maps/acquiring/script-tag-inline.html
index 683ce83c3a4b35..602ddc41a257b1 100644
--- a/import-maps/acquiring/script-tag-inline.html
+++ b/import-maps/acquiring/script-tag-inline.html
@@ -3,18 +3,11 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script>
-const t = async_test(
-  'After inline <script type="module"> import maps should fire error events');
 const log = [];
 </script>
 <script type="module">
-// While this inline module script doesn't have any specifiers and doesn't fetch
-// anything, this still disables subsequent import maps, because
-// https://wicg.github.io/import-maps/#wait-for-import-maps
-// is anyway called at the beginning of
-// https://html.spec.whatwg.org/multipage/webappapis.html#fetch-an-inline-module-script-graph
 </script>
-<script type="importmap" onerror="t.done()">
+<script type="importmap">
 {
   "imports": {
     "../resources/log.js?pipe=sub&name=A": "../resources/log.js?pipe=sub&name=B"
@@ -24,7 +17,7 @@
 <script>
 promise_test(() => {
   return import("../resources/log.js?pipe=sub&name=A")
-    .then(() => assert_array_equals(log, ["log:A"]))
+    .then(() => assert_array_equals(log, ["log:B"]))
   },
-  'After inline <script type="module"> import maps are not effective');
+  'After inline <script type="module"> import maps work fine');
 </script>
diff --git a/import-maps/acquiring/script-tag.html b/import-maps/acquiring/script-tag.html
index 2792c2a31d9718..e03f24dc0afb45 100644
--- a/import-maps/acquiring/script-tag.html
+++ b/import-maps/acquiring/script-tag.html
@@ -3,12 +3,10 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script>
-const t = async_test(
-  'After <script type="module"> import maps should fire error events');
 const log = [];
 </script>
 <script type="module" src="../resources/empty.js?pipe=trickle(d1)"></script>
-<script type="importmap" onerror="t.done()">
+<script type="importmap">
 {
   "imports": {
     "../resources/log.js?pipe=sub&name=A": "../resources/log.js?pipe=sub&name=B"
@@ -18,7 +16,7 @@
 <script>
 promise_test(() => {
   return import("../resources/log.js?pipe=sub&name=A")
-    .then(() => assert_array_equals(log, ["log:A"]))
+    .then(() => assert_array_equals(log, ["log:B"]))
   },
-  'After <script type="module"> import maps are not effective');
+  'After <script type="module"> import maps work fine');
 </script>
diff --git a/import-maps/multiple-import-maps/already-resolved-dropped.html b/import-maps/multiple-import-maps/already-resolved-dropped.html
new file mode 100644
index 00000000000000..631af5dc3ce39a
--- /dev/null
+++ b/import-maps/multiple-import-maps/already-resolved-dropped.html
@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<html>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/import-maps/resources/test-helper.js"></script>
+<script>
+// Simulate resolving a module before import maps are processed
+import("../resources/log.js?pipe=sub&name=ModuleA").catch(() => {});
+</script>
+<script type="importmap">
+{
+   "imports": {
+    "../resources/log.js?pipe=sub&name=ModuleA": "../resources/log.js?pipe=sub&name=ModuleB",
+    "http:/": "../resources/log.js?pipe=sub&name=scheme",
+    "https:/": "../resources/log.js?pipe=sub&name=scheme"
+  }
+}
+</script>
+<script>
+test_loaded(
+  "../resources/log.js?pipe=sub&name=ModuleA",
+  ["log:ModuleA"],
+  "Rules for already resolved modules are dropped"
+);
+</script>
+</html>
diff --git a/import-maps/multiple-import-maps/basic.html b/import-maps/multiple-import-maps/basic.html
index 9ab03ddc302af1..1c55e09e79f42e 100644
--- a/import-maps/multiple-import-maps/basic.html
+++ b/import-maps/multiple-import-maps/basic.html
@@ -22,15 +22,13 @@
 }
 </script>
 <script>
-// Currently the spec doesn't allow multiple import maps, by setting acquiring
-// import maps to false on preparing the first import map.
 promise_test(() => {
   return import("../resources/log.js?pipe=sub&name=A1")
     .then(() => import("../resources/log.js?pipe=sub&name=A2"))
     .then(() => import("../resources/log.js?pipe=sub&name=A3"))
     .then(() => assert_array_equals(
                     log,
-                    ["onerror 2", "log:B1", "log:B2", "log:A3"]))
+                    ["log:B1", "log:B2", "log:C3"]))
   },
-  "Second import map should be rejected");
+  "Second import map should be used for resolution");
 </script>
diff --git a/import-maps/multiple-import-maps/conflict-first-persists.html b/import-maps/multiple-import-maps/conflict-first-persists.html
new file mode 100644
index 00000000000000..2e197244be55c6
--- /dev/null
+++ b/import-maps/multiple-import-maps/conflict-first-persists.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<html>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script type="importmap">
+{
+   "imports": {
+    "module-a": "../resources/log.js?pipe=sub&name=ModuleA",
+    "module-b/something": "../resources/log.js?pipe=sub&name=ModuleB"
+  }
+}
+</script>
+<script type="importmap">
+{
+  "imports": {
+    "module-a": "../resources/log.js?pipe=sub&name=OtherModuleA",
+    "module-b/": "../resources/log.js?pipe=sub&name=PrefixModuleB",
+    "module-b": "../resources/log.js?pipe=sub&name=OtherModuleB"
+  }
+}
+</script>
+<script>
+const test_loaded = (specifier, expected_log, description) => {
+  promise_test(async t => {
+    log = [];
+    await import(specifier);
+    assert_array_equals(log, expected_log);
+  }, description);
+};
+
+test_loaded(
+  "module-a",
+  ["log:ModuleA"],
+  "First defined rule persists in case of conflict"
+);
+
+test_loaded(
+  "module-b/something",
+  ["log:ModuleB"],
+  "First defined rule persists in case of conflict - prefixed bare specifiers"
+);
+
+test_loaded(
+  "module-b",
+  ["log:OtherModuleB"],
+  "First defined rule persists in case of conflict - non-prefix bare specifier"
+);
+</script>
+</html>
diff --git a/import-maps/multiple-import-maps/resolution-consistency-in-module-tree-inline.html b/import-maps/multiple-import-maps/resolution-consistency-in-module-tree-inline.html
new file mode 100644
index 00000000000000..a248977b90dd9e
--- /dev/null
+++ b/import-maps/multiple-import-maps/resolution-consistency-in-module-tree-inline.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script>
+function waitForBool(boolName) {
+  return new Promise((resolve) => {
+    const checkVariable = setInterval(() => {
+      if (window[boolName]) {
+        clearInterval(checkVariable);
+        resolve();
+      }
+    }, 0);
+  });
+}
+
+step_timeout(() => {
+  const importMapScript = document.createElement('script');
+  importMapScript.type = 'importmap';
+  importMapScript.textContent = JSON.stringify({
+    imports: {
+      "../resources/log.sub.js?name=A": "../resources/log.sub.js?name=B"
+    }
+  });
+  document.head.appendChild(importMapScript);
+}, 100);
+const log = [];
+</script>
+<script type="module">
+import "../resources/importer.sub.js?pipe=trickle(d0.5)&name=..%2Fresources%2Flog.sub.js%3Fname%3DA";
+</script>
+<script type="module">
+test(() => {
+  assert_array_equals(log, ["log:B"], "Import should use the new import map");
+}, "Module tree that started to download before a new import map should still take it into account");
+</script>
+</body>
+</html>
+
+
+
+
diff --git a/import-maps/multiple-import-maps/resolution-consistency-in-module-tree.html b/import-maps/multiple-import-maps/resolution-consistency-in-module-tree.html
new file mode 100644
index 00000000000000..e0767a6c61056d
--- /dev/null
+++ b/import-maps/multiple-import-maps/resolution-consistency-in-module-tree.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script type="module" async>
+step_timeout(() => {
+  const importMapScript = document.createElement('script');
+  importMapScript.type = 'importmap';
+  importMapScript.textContent = JSON.stringify({
+    imports: {
+      "../resources/log.sub.js?name=A": "../resources/log.sub.js?name=B"
+    }
+  });
+  document.head.appendChild(importMapScript);
+}, 100);
+</script>
+<script>
+const log = [];
+</script>
+<script type="module" src="../resources/importer.sub.js?pipe=trickle(d0.5)&name=..%2Fresources%2Flog.sub.js%3Fname%3DA"></script>
+<script type="module">
+test(() => {
+  assert_array_equals(log, ["log:B"], "Import should use the new import map");
+}, "Module tree that started to download before a new import map should still take it into account");
+</script>
+</body>
+</html>
+
+
+
diff --git a/import-maps/multiple-import-maps/url-resolution-conflict.html b/import-maps/multiple-import-maps/url-resolution-conflict.html
new file mode 100644
index 00000000000000..e8d06bdf524897
--- /dev/null
+++ b/import-maps/multiple-import-maps/url-resolution-conflict.html
@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<html>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+const log = [];
+</script>
+<script type="importmap">
+{
+  "scopes": {
+    "/": {
+      "../resources/../resources/app.js": "../resources/log.js?pipe=sub&name=first"
+    }
+  }
+}
+</script>
+<script type="importmap">
+{
+  "scopes": {
+    "/": {
+      "../resources/app.js": "../resources/log.js?pipe=sub&name=second"
+    }
+  }
+}
+</script>
+<script type="module">
+promise_test(async () => {
+  await import("../resources/app.js");
+  assert_array_equals(log, ["log:first"]);
+},
+"Second import map should not override same resolved URL");
+</script>
diff --git a/import-maps/multiple-import-maps/with-errors.html b/import-maps/multiple-import-maps/with-errors.html
index a93ecaaeff5b32..3ba9b073592532 100644
--- a/import-maps/multiple-import-maps/with-errors.html
+++ b/import-maps/multiple-import-maps/with-errors.html
@@ -18,15 +18,11 @@
 }
 </script>
 <script>
-// Currently the spec doesn't allow multiple import maps, by setting acquiring
-// import maps to false on preparing the first import map.
-// Even the first import map has errors and thus Document's import map is not
-// updated, the second import map is still rejected at preparationg.
 promise_test(() => {
   return import("../resources/log.js?pipe=sub&name=A")
     .then(() => assert_array_equals(
                     log,
-                    ["onerror 2", "log:A"]))
+                    ["log:C"]))
   },
-  "Second import map should be rejected after an import map with errors");
+  "Second import map should be used for resolution even after an import map with errors");
 </script>
diff --git a/import-maps/not-overridden/dynamic.html b/import-maps/not-overridden/dynamic.html
new file mode 100644
index 00000000000000..dbc839a928c0cb
--- /dev/null
+++ b/import-maps/not-overridden/dynamic.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script>
+const log = [];
+
+// First, use a module specifier
+promise_test(() => {
+  return import("../resources/log.js?pipe=sub&name=A")
+    .then(() => {
+      assert_array_equals(log, ["log:A"], "First import should use original module");
+    });
+}, "Initial import before import map");
+
+// Now try to redefine it with multiple import map rules
+</script>
+<script type="importmap">
+{
+  "imports": {
+    "../resources/log.js?pipe=sub&name=A": "../resources/log.js?pipe=sub&name=B",
+    "http:/": "../resources/log.js?pipe=sub&name=scheme",
+    "https:/": "../resources/log.js?pipe=sub&name=scheme"
+  }
+}
+</script>
+<script type="module">
+// Testing that the resolution is correct using `resolve`, as you can't import
+// the same module twice.
+test(() => {
+  assert_true(import.meta.resolve("../resources/log.js?pipe=sub&name=A")
+              .endsWith("/resources/log.js?pipe=sub&name=A"));
+}, "Resolution after import map should not be redefined");
+</script>
+</body>
+</html>
+
diff --git a/import-maps/not-overridden/external-script-bare-descendent.html b/import-maps/not-overridden/external-script-bare-descendent.html
new file mode 100644
index 00000000000000..bff33f8d6abc2b
--- /dev/null
+++ b/import-maps/not-overridden/external-script-bare-descendent.html
@@ -0,0 +1,52 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script type="importmap">
+{
+  "imports": {
+    "a": "../resources/log.sub.js?name=A",
+    "foo/bar": "../resources/log.sub.js?name=FOOBAR"
+  }
+}
+</script>
+<script type="module" src="../resources/importer.sub.js?name=a"></script>
+<script type="module" src="../resources/importer.sub.js?name=foo/bar"></script>
+<script>
+const log = [];
+
+// Try to redefine the descendent with an import map
+</script>
+<script type="importmap">
+{
+  "imports": {
+    "a": "../resources/log.sub.js?name=B",
+    "foo/bar": "../resources/log.sub.js?name=OtherFoobar",
+    "foo/": "../resources/log.sub.js?name=OtherFoo",
+    "foo": "../resources/log.sub.js?name=foo"
+  }
+}
+</script>
+<script type="module">
+// Testing that the resolution is correct using `resolve`, as you can't import
+// the same module twice.
+test(() => {
+  assert_true(import.meta.resolve("a").endsWith("/resources/log.sub.js?name=A"));
+}, "Resolution after import map should not be redefined");
+
+test(() => {
+  assert_true(import.meta.resolve("foo/bar").endsWith("/resources/log.sub.js?name=FOOBAR"));
+}, "Resolution after import map should not be redefined for bare prefixes or exact matches");
+
+test(() => {
+  assert_true(import.meta.resolve("foo").endsWith("/resources/log.sub.js?name=foo"));
+}, "Resolution after import map should be redefined for non-prefixes");
+</script>
+</body>
+</html>
+
+
+
diff --git a/import-maps/not-overridden/external-script-descendent-with-scope.html b/import-maps/not-overridden/external-script-descendent-with-scope.html
new file mode 100644
index 00000000000000..9f04548cc2d749
--- /dev/null
+++ b/import-maps/not-overridden/external-script-descendent-with-scope.html
@@ -0,0 +1,81 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script type="importmap">
+{
+  "imports": {
+    "a": "../resources/log.sub.js?name=A"
+  }
+}
+</script>
+<script type="module" src="../resources/importer.sub.js?name=a"></script>
+<script>
+const variable_defined = (variable) => {
+  return new Promise((resolve) => {
+    const interval = setInterval(() => {
+      if (eval(variable) !== undefined) {
+        clearInterval(interval);
+        resolve(variable);
+      }
+    }, 10);
+  });
+};
+const log = [];
+promise_test(async () => {
+  await variable_defined("log[0]");
+  // Try to redefine the descendent with an import map
+  const importMapScript = document.createElement('script');
+  importMapScript.type = 'importmap';
+  importMapScript.textContent =
+`{
+  "scopes": {
+    "/import-maps/resources/": {
+      "a": "../resources/log.sub.js?name=B"
+    },
+    "/resources/": {
+      "a": "../resources/log.sub.js?name=D",
+       "../resources/": "/foobar/"
+    }
+  },
+  "imports": {
+    "a": "../resources/log.sub.js?name=C"
+  }
+}`;
+  document.head.appendChild(importMapScript);
+
+  const inScopeModule = document.createElement('script');
+  inScopeModule.src = "../resources/in-scope-test.js";
+  inScopeModule.type = 'module';
+  document.head.appendChild(inScopeModule);
+  await variable_defined("window.inscope_test_result");
+  assert_true(window.inscope_test_result
+              .endsWith("/resources/log.sub.js?name=A"), "inscope");
+
+  const outOfScopeModule = document.createElement('script');
+  outOfScopeModule.src = "/resources/out-of-scope-test.js";
+  outOfScopeModule.type = 'module';
+  document.head.appendChild(outOfScopeModule);
+  await variable_defined("window.outscope_test_result");
+  assert_true(window.outscope_test_result
+              .endsWith("/resources/log.sub.js?name=D"), "out of scope 1");
+  assert_true(window.outscope_test_result2
+              .endsWith("/resources/log.sub.js?name=E"), "out of scope 2");
+
+  const inlineModule = document.createElement('script');
+  inlineModule.type = 'module';
+  inlineModule.innerHTML = `window.inline_test_result = import.meta.resolve("a");`;
+  document.head.appendChild(inlineModule);
+  await variable_defined("window.inline_test_result");
+  assert_true(window.inline_test_result
+              .endsWith("/resources/log.sub.js?name=A"), "inline");
+}, "Testing descendent resolution with scopes");
+</script>
+</body>
+</html>
+
+
+
diff --git a/import-maps/not-overridden/external-script-descendent.html b/import-maps/not-overridden/external-script-descendent.html
new file mode 100644
index 00000000000000..122e8204f18fce
--- /dev/null
+++ b/import-maps/not-overridden/external-script-descendent.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script type="module" src="../resources/importer.sub.js?name=..%2Fresources%2Flog.sub.js%3Fname%3DA"></script>
+<script>
+const log = [];
+(async (variable) => {
+  await new Promise((resolve) => {
+    const interval = setInterval(() => {
+      if (variable.length > 0) {
+        clearInterval(interval);
+        resolve(variable);
+      }
+    }, 10);
+  });
+  const importMapScript = document.createElement('script');
+  importMapScript.type = 'importmap';
+  importMapScript.textContent = JSON.stringify({
+    imports: {
+      "../resources/log.sub.js?name=A": "../resources/log.sub.js?name=B"
+    }
+  });
+  document.head.appendChild(importMapScript);
+
+  const inlineModule = document.createElement('script');
+  inlineModule.type = 'module';
+  inlineModule.innerHTML = `test(() => {
+  assert_true(import.meta.resolve("../resources/log.sub.js?name=A").endsWith("/resources/log.sub.js?name=A"));
+}, "Resolution after import map should not be redefined");`;
+  document.head.appendChild(inlineModule);
+})(log);
+</script>
+</body>
+</html>
+
+
diff --git a/import-maps/not-overridden/failed-resolution.html b/import-maps/not-overridden/failed-resolution.html
new file mode 100644
index 00000000000000..eae6febddf157b
--- /dev/null
+++ b/import-maps/not-overridden/failed-resolution.html
@@ -0,0 +1,60 @@
+<!DOCTYPE html>
+<html>
+<head>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/import-maps/resources/test-helper.js"></script>
+</head>
+<body>
+<script type="module">
+const log = [];
+// Import specifiers that were not yet mapped.
+try {
+  await import("../resources/importer.sub.js?name=a")
+} catch (error) {
+  // Import failed because "a" is not yet defined as a specifier.
+}
+try {
+  await import("../resources/importer.sub.js?name=foo/bar")
+} catch (error) {
+  // Import failed because "foo/bar" is not yet defined as a specifier.
+}
+// Try to define the previous failed-to-resolve specifiers.
+const importMapScript = document.createElement('script');
+importMapScript.type = 'importmap';
+importMapScript.textContent =
+`{
+  "imports": {
+    "a": "../resources/log.sub.js?name=B",
+    "foo/bar": "../resources/log.sub.js?name=OtherFoobar",
+    "foo/": "../resources/log.sub.js?name=OtherFoo/",
+    "foo": "../resources/log.sub.js?name=foo"
+  }
+}`;
+document.head.appendChild(importMapScript);
+
+// Testing that the resolution is correct using `resolve`, as you can't import
+// the same module twice.
+test(() => {
+  assert_true(import.meta.resolve("a").endsWith("/resources/log.sub.js?name=B"));
+}, "Resolution after import map should not be redefined");
+
+test(() => {
+  assert_true(import.meta.resolve("foo/bar").endsWith("/resources/log.sub.js?name=OtherFoobar"));
+}, "Resolution after import map should not be redefined for bare prefixes or exact matches");
+
+test(() => {
+  assert_true(import.meta.resolve("foo").endsWith("/resources/log.sub.js?name=foo"));
+}, "Resolution after import map should be redefined for non-prefixes");
+
+test_loaded(
+  "../resources/log.sub.js?name=a",
+  ["log:a"],
+  "Rules for failed resolution are not dropped"
+);
+</script>
+</body>
+</html>
+
+
+
diff --git a/import-maps/not-overridden/integrity.html b/import-maps/not-overridden/integrity.html
new file mode 100644
index 00000000000000..7770899728706c
--- /dev/null
+++ b/import-maps/not-overridden/integrity.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script>
+const log = [];
+</script>
+<script type="importmap">
+{
+  "imports": {
+    "../resources/log.js?pipe=sub&name=A": "../resources/log.js?pipe=sub&name=B"
+  },
+  "integrity": {
+    "../resources/log.js?pipe=sub&name=B": "sha384-PeZ0Scqs60QjpuvHAyomddrpcCuox9hUwAff1yx1Lv2HtPIAfsSuHi6VKGK3nH3w"
+  }
+}
+</script>
+<script type="importmap">
+{
+  "integrity": {
+    "../resources/log.js?pipe=sub&name=B": "sha384-foobar"
+  }
+}
+</script>
+<script type="module">
+import '../resources/log.js?pipe=sub&name=A';
+</script>
+<script type="module">
+test(t => {
+  assert_array_equals(log, ["log:B"]);
+}, 'Static script loaded as its integrity check passed');
+</script>
+</body>
+</html>
+
diff --git a/import-maps/not-overridden/modulepreload-header.html.headers b/import-maps/not-overridden/modulepreload-header.html.headers
new file mode 100644
index 00000000000000..785378622d4d81
--- /dev/null
+++ b/import-maps/not-overridden/modulepreload-header.html.headers
@@ -0,0 +1,2 @@
+Link: <../resources/log.js?pipe=sub&name=A>;rel=modulepreload
+
diff --git a/import-maps/not-overridden/prefix.html b/import-maps/not-overridden/prefix.html
new file mode 100644
index 00000000000000..99449b890e6021
--- /dev/null
+++ b/import-maps/not-overridden/prefix.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script>
+const log = [];
+
+// First, use a module specifier
+promise_test(() => {
+  return import("../resources/log.js?pipe=sub&name=A")
+    .then(() => {
+      assert_array_equals(log, ["log:A"], "First import should use original module");
+    });
+}, "Initial import before import map");
+
+// Now try to redefine it with an import map
+</script>
+<script type="importmap">
+{
+  "imports": {
+    "../resources/": "/foobar/"
+  }
+}
+</script>
+<script type="module">
+// Testing that the resolution is correct using `resolve`, as you can't import
+// the same module twice.
+test(() => {
+  assert_false(import.meta.resolve("../resources/log.js?pipe=sub&name=A")
+              .endsWith("/foobar/log.js?pipe=sub&name=A"));
+  assert_true(import.meta.resolve("../resources/log.js?pipe=sub&name=A")
+              .endsWith("/resources/log.js?pipe=sub&name=A"));
+}, "Prefix resolution after import map should not be redefined");
+</script>
+</body>
+</html>
+
diff --git a/import-maps/not-overridden/script-descendent.html b/import-maps/not-overridden/script-descendent.html
new file mode 100644
index 00000000000000..4d1e08d41298e4
--- /dev/null
+++ b/import-maps/not-overridden/script-descendent.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+<script type="module">
+import "../resources/log.js?pipe=sub&name=A";
+</script>
+<script>
+const log = [];
+
+// Try to redefine the descendent with an import map
+</script>
+<script type="importmap">
+{
+  "imports": {
+    "../resources/log.js?pipe=sub&name=A": "../resources/log.js?pipe=sub&name=B"
+  }
+}
+</script>
+<script type="module">
+// Testing that the resolution is correct using `resolve`, as you can't import
+// the same module twice.
+test(() => {
+  assert_true(import.meta.resolve("../resources/log.js?pipe=sub&name=A")
+              .endsWith("/resources/log.js?pipe=sub&name=A"));
+}, "Resolution after import map should not be redefined");
+</script>
+</body>
+</html>
+
diff --git a/import-maps/not-overridden/url-resolution-conflict.html b/import-maps/not-overridden/url-resolution-conflict.html
new file mode 100644
index 00000000000000..d0708632164393
--- /dev/null
+++ b/import-maps/not-overridden/url-resolution-conflict.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<html>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+const log = [];
+</script>
+</script>
+<script type="module">
+import '../resources/../resources/log.js?pipe=sub&name=a';
+</script>
+<script type="importmap">
+{
+  "scopes": {
+    "/": {
+      "../resources/log.js?pipe=sub&name=a": "../resources/log.js?pipe=sub&name=b"
+    }
+  }
+}
+</script>
+<script type="module">
+promise_test(async () => {
+  await import("../resources/log.js?pipe=sub&name=a");
+  assert_array_equals(log, ["log:a"]);
+},
+"import map should not override already resolved URL, even if spelled differently");
+</script>
+
diff --git a/import-maps/resources/importer.sub.js b/import-maps/resources/importer.sub.js
new file mode 100644
index 00000000000000..6b55546d0d9a5c
--- /dev/null
+++ b/import-maps/resources/importer.sub.js
@@ -0,0 +1 @@
+import "{{GET[name]}}";
diff --git a/import-maps/resources/in-scope-test.js b/import-maps/resources/in-scope-test.js
new file mode 100644
index 00000000000000..aa64e89514c698
--- /dev/null
+++ b/import-maps/resources/in-scope-test.js
@@ -0,0 +1,3 @@
+// Testing that the resolution is correct using `resolve`, as you can't import
+// the same module twice.
+window.inscope_test_result = import.meta.resolve("a");
diff --git a/import-maps/resources/log.sub.js b/import-maps/resources/log.sub.js
new file mode 100644
index 00000000000000..3588867d52cc96
--- /dev/null
+++ b/import-maps/resources/log.sub.js
@@ -0,0 +1,2 @@
+log.push("log:{{GET[name]}}");
+
diff --git a/import-maps/resources/test-helper.js b/import-maps/resources/test-helper.js
index f12e84c685d548..258d8f4afe93f5 100644
--- a/import-maps/resources/test-helper.js
+++ b/import-maps/resources/test-helper.js
@@ -244,3 +244,11 @@ function doTests(importMapString, importMapBaseURL, tests) {
     });
   }, { explicit_done: true });
 }
+
+function test_loaded(specifier, expected_log, description) {
+  promise_test(async t => {
+    log = [];
+    await import(specifier);
+    assert_array_equals(log, expected_log);
+  }, description);
+};
diff --git a/resources/out-of-scope-test.js b/resources/out-of-scope-test.js
new file mode 100644
index 00000000000000..ce24124646dc83
--- /dev/null
+++ b/resources/out-of-scope-test.js
@@ -0,0 +1,5 @@
+// Testing that the resolution is correct using `resolve`, as you can't import
+// the same module twice.
+window.outscope_test_result = import.meta.resolve("a");
+window.outscope_test_result2 = import.meta.resolve("../resources/log.sub.js?name=E");
+