Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

24 files changed, 9165 insertions, 0 deletions

diff --git a/dom/script/AutoEntryScript.cpp b/dom/script/AutoEntryScript.cpp
new file mode 100644
index 0000000000..ed58e3a1fd
--- /dev/null
+++ b/dom/script/AutoEntryScript.cpp
@@ -0,0 +1,82 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/dom/AutoEntryScript.h"
+
+#include <stdint.h>
+#include <utility>
+#include "js/ProfilingCategory.h"
+#include "js/ProfilingStack.h"
+#include "jsapi.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+#include "nsCOMPtr.h"
+#include "nsContentUtils.h"
+#include "nsGlobalWindowInner.h"
+#include "nsIDocShell.h"
+#include "nsJSUtils.h"
+#include "nsPIDOMWindow.h"
+#include "nsPIDOMWindowInlines.h"
+#include "nsString.h"
+#include "xpcpublic.h"
+
+namespace mozilla::dom {
+
+namespace {
+// Assert if it's not safe to run script. The helper class
+// AutoAllowLegacyScriptExecution allows to allow-list
+// legacy cases where it's actually not safe to run script.
+#ifdef DEBUG
+void AssertIfNotSafeToRunScript() {
+  // if it's safe to run script, then there is nothing to do here.
+  if (nsContentUtils::IsSafeToRunScript()) {
+    return;
+  }
+
+  // auto allowing legacy script execution is fine for now.
+  if (AutoAllowLegacyScriptExecution::IsAllowed()) {
+    return;
+  }
+
+  MOZ_ASSERT(false, "is it safe to run script?");
+}
+#endif
+
+}  // namespace
+
+AutoEntryScript::AutoEntryScript(nsIGlobalObject* aGlobalObject,
+                                 const char* aReason, bool aIsMainThread)
+    : AutoJSAPI(aGlobalObject, aIsMainThread, eEntryScript),
+      mWebIDLCallerPrincipal(nullptr)
+      // This relies on us having a cx() because the AutoJSAPI constructor
+      // already ran.
+      ,
+      mCallerOverride(cx()),
+      mAutoProfilerLabel(
+          "", aReason, JS::ProfilingCategoryPair::JS,
+          uint32_t(js::ProfilingStackFrame::Flags::RELEVANT_FOR_JS)),
+      mJSThreadExecution(aGlobalObject, aIsMainThread) {
+  MOZ_ASSERT(aGlobalObject);
+
+  if (aIsMainThread) {
+#ifdef DEBUG
+    AssertIfNotSafeToRunScript();
+#endif
+    mScriptActivity.emplace(true);
+  }
+}
+
+AutoEntryScript::AutoEntryScript(JSObject* aObject, const char* aReason,
+                                 bool aIsMainThread)
+    : AutoEntryScript(xpc::NativeGlobal(aObject), aReason, aIsMainThread) {
+  // xpc::NativeGlobal uses JS::GetNonCCWObjectGlobal, which asserts that
+  // aObject is not a CCW.
+}
+
+AutoEntryScript::~AutoEntryScript() = default;
+
+}  // namespace mozilla::dom
diff --git a/dom/script/AutoEntryScript.h b/dom/script/AutoEntryScript.h
new file mode 100644
index 0000000000..905baf0857
--- /dev/null
+++ b/dom/script/AutoEntryScript.h
@@ -0,0 +1,78 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef DOM_SCRIPT_AUTOENTRYSCRIPT_H_
+#define DOM_SCRIPT_AUTOENTRYSCRIPT_H_
+
+#include "MainThreadUtils.h"
+#include "js/Debug.h"
+#include "js/TypeDecls.h"
+#include "jsapi.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/dom/JSExecutionManager.h"
+#include "mozilla/dom/ScriptSettings.h"
+
+class nsIGlobalObject;
+class nsIPrincipal;
+
+namespace xpc {
+class AutoScriptActivity;
+}
+
+namespace mozilla::dom {
+
+/*
+ * A class that represents a new script entry point.
+ *
+ * |aReason| should be a statically-allocated C string naming the reason we're
+ * invoking JavaScript code: "setTimeout", "event", and so on. The devtools use
+ * these strings to label JS execution in timeline and profiling displays.
+ *
+ */
+class MOZ_STACK_CLASS AutoEntryScript : public AutoJSAPI {
+ public:
+  // Constructing the AutoEntryScript will ensure that it enters the
+  // Realm of aGlobalObject's JSObject and exposes that JSObject to active JS.
+  AutoEntryScript(nsIGlobalObject* aGlobalObject, const char* aReason,
+                  bool aIsMainThread = NS_IsMainThread());
+
+  // aObject can be any object from the relevant global. It must not be a
+  // cross-compartment wrapper because CCWs are not associated with a single
+  // global.
+  //
+  // Constructing the AutoEntryScript will ensure that it enters the
+  // Realm of aObject JSObject and exposes aObject's global to active JS.
+  AutoEntryScript(JSObject* aObject, const char* aReason,
+                  bool aIsMainThread = NS_IsMainThread());
+
+  ~AutoEntryScript();
+
+  void SetWebIDLCallerPrincipal(nsIPrincipal* aPrincipal) {
+    mWebIDLCallerPrincipal = aPrincipal;
+  }
+
+ private:
+  // It's safe to make this a weak pointer, since it's the subject principal
+  // when we go on the stack, so can't go away until after we're gone.  In
+  // particular, this is only used from the CallSetup constructor, and only in
+  // the aIsJSImplementedWebIDL case.  And in that case, the subject principal
+  // is the principal of the callee function that is part of the CallArgs just a
+  // bit up the stack, and which will outlive us.  So we know the principal
+  // can't go away until then either.
+  nsIPrincipal* MOZ_NON_OWNING_REF mWebIDLCallerPrincipal;
+  friend nsIPrincipal* GetWebIDLCallerPrincipal();
+
+  Maybe<xpc::AutoScriptActivity> mScriptActivity;
+  JS::AutoHideScriptedCaller mCallerOverride;
+  AutoProfilerLabel mAutoProfilerLabel;
+  AutoRequestJSThreadExecution mJSThreadExecution;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // DOM_SCRIPT_AUTOENTRYSCRIPT_H_
diff --git a/dom/script/ModuleLoader.cpp b/dom/script/ModuleLoader.cpp
new file mode 100644
index 0000000000..5d6b90cbf0
--- /dev/null
+++ b/dom/script/ModuleLoader.cpp
@@ -0,0 +1,371 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ScriptLoader.h"
+#include "ModuleLoader.h"
+
+#include "jsapi.h"
+#include "js/CompileOptions.h"  // JS::CompileOptions, JS::InstantiateOptions
+#include "js/ContextOptions.h"  // JS::ContextOptionsRef
+#include "js/experimental/JSStencil.h"  // JS::Stencil, JS::CompileModuleScriptToStencil, JS::InstantiateModuleStencil
+#include "js/MemoryFunctions.h"
+#include "js/Modules.h"  // JS::FinishDynamicModuleImport, JS::{G,S}etModuleResolveHook, JS::Get{ModulePrivate,ModuleScript,RequestedModule{s,Specifier,SourcePos}}, JS::SetModule{DynamicImport,Metadata}Hook
+#include "js/PropertyAndElement.h"  // JS_DefineProperty
+#include "js/Realm.h"
+#include "js/SourceText.h"
+#include "js/loader/LoadedScript.h"
+#include "js/loader/ScriptLoadRequest.h"
+#include "js/loader/ModuleLoaderBase.h"
+#include "js/loader/ModuleLoadRequest.h"
+#include "mozilla/dom/RequestBinding.h"
+#include "mozilla/Assertions.h"
+#include "nsError.h"
+#include "xpcpublic.h"
+#include "GeckoProfiler.h"
+#include "nsContentSecurityManager.h"
+#include "nsIContent.h"
+#include "nsJSUtils.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/dom/AutoEntryScript.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Element.h"
+#include "nsIPrincipal.h"
+#include "mozilla/LoadInfo.h"
+#include "mozilla/Maybe.h"
+
+using JS::SourceText;
+using namespace JS::loader;
+
+namespace mozilla::dom {
+
+#undef LOG
+#define LOG(args) \
+  MOZ_LOG(ScriptLoader::gScriptLoaderLog, mozilla::LogLevel::Debug, args)
+
+#define LOG_ENABLED() \
+  MOZ_LOG_TEST(ScriptLoader::gScriptLoaderLog, mozilla::LogLevel::Debug)
+
+//////////////////////////////////////////////////////////////
+// DOM module loader
+//////////////////////////////////////////////////////////////
+
+ModuleLoader::ModuleLoader(ScriptLoader* aLoader,
+                           nsIGlobalObject* aGlobalObject, Kind aKind)
+    : ModuleLoaderBase(aLoader, aGlobalObject), mKind(aKind) {}
+
+ScriptLoader* ModuleLoader::GetScriptLoader() {
+  return static_cast<ScriptLoader*>(mLoader.get());
+}
+
+bool ModuleLoader::CanStartLoad(ModuleLoadRequest* aRequest, nsresult* aRvOut) {
+  if (!GetScriptLoader()->GetDocument()) {
+    *aRvOut = NS_ERROR_NULL_POINTER;
+    return false;
+  }
+
+  // If this document is sandboxed without 'allow-scripts', abort.
+  if (GetScriptLoader()->GetDocument()->HasScriptsBlockedBySandbox()) {
+    *aRvOut = NS_OK;
+    return false;
+  }
+
+  // To prevent dynamic code execution, content scripts can only
+  // load moz-extension URLs.
+  nsCOMPtr<nsIPrincipal> principal = aRequest->TriggeringPrincipal();
+  if (BasePrincipal::Cast(principal)->ContentScriptAddonPolicy() &&
+      !aRequest->mURI->SchemeIs("moz-extension")) {
+    *aRvOut = NS_ERROR_DOM_WEBEXT_CONTENT_SCRIPT_URI;
+    return false;
+  }
+
+  if (LOG_ENABLED()) {
+    nsAutoCString url;
+    aRequest->mURI->GetAsciiSpec(url);
+    LOG(("ScriptLoadRequest (%p): Start Module Load (url = %s)", aRequest,
+         url.get()));
+  }
+
+  return true;
+}
+
+nsresult ModuleLoader::StartFetch(ModuleLoadRequest* aRequest) {
+  // According to the spec, module scripts have different behaviour to classic
+  // scripts and always use CORS. Only exception: Non linkable about: pages
+  // which load local module scripts.
+  bool isAboutPageLoadingChromeURI = ScriptLoader::IsAboutPageLoadingChromeURI(
+      aRequest, GetScriptLoader()->GetDocument());
+
+  nsContentSecurityManager::CORSSecurityMapping corsMapping =
+      isAboutPageLoadingChromeURI
+          ? nsContentSecurityManager::CORSSecurityMapping::DISABLE_CORS_CHECKS
+          : nsContentSecurityManager::CORSSecurityMapping::REQUIRE_CORS_CHECKS;
+
+  nsSecurityFlags securityFlags =
+      nsContentSecurityManager::ComputeSecurityFlags(aRequest->CORSMode(),
+                                                     corsMapping);
+
+  securityFlags |= nsILoadInfo::SEC_ALLOW_CHROME;
+
+  // Delegate Shared Behavior to base ScriptLoader
+  //
+  // aCharsetForPreload is passed as Nothing() because this is not a preload
+  // and `StartLoadInternal` is able to find the charset by using `aRequest`
+  // for this case.
+  nsresult rv = GetScriptLoader()->StartLoadInternal(
+      aRequest, securityFlags, Nothing() /* aCharsetForPreload */);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // https://html.spec.whatwg.org/multipage/webappapis.html#fetch-an-import()-module-script-graph
+  // Step 1. Disallow further import maps given settings object.
+  if (!aRequest->GetScriptLoadContext()->IsPreload()) {
+    LOG(("ScriptLoadRequest (%p): Disallow further import maps.", aRequest));
+    DisallowImportMaps();
+  }
+
+  LOG(("ScriptLoadRequest (%p): Start fetching module", aRequest));
+
+  return NS_OK;
+}
+
+void ModuleLoader::AsyncExecuteInlineModule(ModuleLoadRequest* aRequest) {
+  MOZ_ALWAYS_SUCCEEDS(NS_DispatchToMainThread(
+      mozilla::NewRunnableMethod<RefPtr<ModuleLoadRequest>>(
+          "ModuleLoader::ExecuteInlineModule", this,
+          &ModuleLoader::ExecuteInlineModule, aRequest)));
+}
+
+void ModuleLoader::ExecuteInlineModule(ModuleLoadRequest* aRequest) {
+  MOZ_ASSERT(aRequest->IsFinished());
+  MOZ_ASSERT(aRequest->IsTopLevel());
+  MOZ_ASSERT(aRequest->GetScriptLoadContext()->mIsInline);
+
+  if (aRequest->GetScriptLoadContext()->GetParserCreated() == NOT_FROM_PARSER) {
+    GetScriptLoader()->RunScriptWhenSafe(aRequest);
+  } else {
+    GetScriptLoader()->MaybeMoveToLoadedList(aRequest);
+    GetScriptLoader()->ProcessPendingRequests();
+  }
+
+  aRequest->GetScriptLoadContext()->MaybeUnblockOnload();
+}
+
+void ModuleLoader::OnModuleLoadComplete(ModuleLoadRequest* aRequest) {
+  MOZ_ASSERT(aRequest->IsFinished());
+
+  if (aRequest->IsTopLevel()) {
+    if (aRequest->GetScriptLoadContext()->mIsInline &&
+        aRequest->GetScriptLoadContext()->GetParserCreated() ==
+            NOT_FROM_PARSER) {
+      if (aRequest->mImports.Length() == 0) {
+        GetScriptLoader()->RunScriptWhenSafe(aRequest);
+      } else {
+        AsyncExecuteInlineModule(aRequest);
+        return;
+      }
+    } else if (aRequest->GetScriptLoadContext()->mIsInline &&
+               aRequest->GetScriptLoadContext()->GetParserCreated() !=
+                   NOT_FROM_PARSER &&
+               !nsContentUtils::IsSafeToRunScript()) {
+      // Avoid giving inline async module scripts that don't have
+      // external dependencies a guaranteed execution time relative
+      // to the HTML parse. That is, deliberately avoid guaranteeing
+      // that the script would always observe a DOM shape where the
+      // parser has not added further elements to the DOM.
+      // (If `nsContentUtils::IsSafeToRunScript()` returns `true`,
+      // we come here synchronously from the parser. If it returns
+      // `false` we come here from an external dependency completing
+      // its fetch, in which case we already are at an unspecific
+      // point relative to the parse.)
+      AsyncExecuteInlineModule(aRequest);
+      return;
+    } else {
+      GetScriptLoader()->MaybeMoveToLoadedList(aRequest);
+      GetScriptLoader()->ProcessPendingRequestsAsync();
+    }
+  }
+
+  aRequest->GetScriptLoadContext()->MaybeUnblockOnload();
+}
+
+nsresult ModuleLoader::CompileFetchedModule(
+    JSContext* aCx, JS::Handle<JSObject*> aGlobal, JS::CompileOptions& aOptions,
+    ModuleLoadRequest* aRequest, JS::MutableHandle<JSObject*> aModuleOut) {
+  if (aRequest->GetScriptLoadContext()->mWasCompiledOMT) {
+    JS::InstantiationStorage storage;
+    RefPtr<JS::Stencil> stencil =
+        aRequest->GetScriptLoadContext()->StealOffThreadResult(aCx, &storage);
+    if (!stencil) {
+      return NS_ERROR_FAILURE;
+    }
+
+    JS::InstantiateOptions instantiateOptions(aOptions);
+    aModuleOut.set(JS::InstantiateModuleStencil(aCx, instantiateOptions,
+                                                stencil, &storage));
+    if (!aModuleOut) {
+      return NS_ERROR_FAILURE;
+    }
+
+    if (aRequest->IsTextSource() &&
+        ScriptLoader::ShouldCacheBytecode(aRequest)) {
+      if (!JS::StartIncrementalEncoding(aCx, std::move(stencil))) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+
+    return NS_OK;
+  }
+
+  if (!nsJSUtils::IsScriptable(aGlobal)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  RefPtr<JS::Stencil> stencil;
+  if (aRequest->IsTextSource()) {
+    MaybeSourceText maybeSource;
+    nsresult rv = aRequest->GetScriptSource(aCx, &maybeSource,
+                                            aRequest->mLoadContext.get());
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    auto compile = [&](auto& source) {
+      return JS::CompileModuleScriptToStencil(aCx, aOptions, source);
+    };
+    stencil = maybeSource.mapNonEmpty(compile);
+  } else {
+    MOZ_ASSERT(aRequest->IsBytecode());
+    JS::DecodeOptions decodeOptions(aOptions);
+    decodeOptions.borrowBuffer = true;
+
+    JS::TranscodeRange range = aRequest->Bytecode();
+    JS::TranscodeResult tr =
+        JS::DecodeStencil(aCx, decodeOptions, range, getter_AddRefs(stencil));
+    if (tr != JS::TranscodeResult::Ok) {
+      return NS_ERROR_DOM_JS_DECODING_ERROR;
+    }
+  }
+
+  if (!stencil) {
+    return NS_ERROR_FAILURE;
+  }
+
+  JS::InstantiateOptions instantiateOptions(aOptions);
+  aModuleOut.set(
+      JS::InstantiateModuleStencil(aCx, instantiateOptions, stencil));
+  if (!aModuleOut) {
+    return NS_ERROR_FAILURE;
+  }
+
+  if (aRequest->IsTextSource() && ScriptLoader::ShouldCacheBytecode(aRequest)) {
+    if (!JS::StartIncrementalEncoding(aCx, std::move(stencil))) {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  return NS_OK;
+}
+
+/* static */
+already_AddRefed<ModuleLoadRequest> ModuleLoader::CreateTopLevel(
+    nsIURI* aURI, ReferrerPolicy aReferrerPolicy,
+    ScriptFetchOptions* aFetchOptions, const SRIMetadata& aIntegrity,
+    nsIURI* aReferrer, ScriptLoader* aLoader, ScriptLoadContext* aContext) {
+  RefPtr<ModuleLoadRequest> request = new ModuleLoadRequest(
+      aURI, aReferrerPolicy, aFetchOptions, aIntegrity, aReferrer, aContext,
+      true,
+      /* is top level */ false, /* is dynamic import */
+      aLoader->GetModuleLoader(),
+      ModuleLoadRequest::NewVisitedSetForTopLevelImport(aURI), nullptr);
+
+  request->NoCacheEntryFound();
+  return request.forget();
+}
+
+already_AddRefed<ModuleLoadRequest> ModuleLoader::CreateStaticImport(
+    nsIURI* aURI, ModuleLoadRequest* aParent) {
+  RefPtr<ScriptLoadContext> newContext = new ScriptLoadContext();
+  newContext->mIsInline = false;
+  // Propagated Parent values. TODO: allow child modules to use root module's
+  // script mode.
+  newContext->mScriptMode = aParent->GetScriptLoadContext()->mScriptMode;
+
+  RefPtr<ModuleLoadRequest> request = new ModuleLoadRequest(
+      aURI, aParent->ReferrerPolicy(), aParent->mFetchOptions, SRIMetadata(),
+      aParent->mURI, newContext, false, /* is top level */
+      false,                            /* is dynamic import */
+      aParent->mLoader, aParent->mVisitedSet, aParent->GetRootModule());
+
+  request->NoCacheEntryFound();
+  return request.forget();
+}
+
+already_AddRefed<ModuleLoadRequest> ModuleLoader::CreateDynamicImport(
+    JSContext* aCx, nsIURI* aURI, LoadedScript* aMaybeActiveScript,
+    JS::Handle<JSString*> aSpecifier, JS::Handle<JSObject*> aPromise) {
+  MOZ_ASSERT(aSpecifier);
+  MOZ_ASSERT(aPromise);
+
+  RefPtr<ScriptFetchOptions> options = nullptr;
+  nsIURI* baseURL = nullptr;
+  RefPtr<ScriptLoadContext> context = new ScriptLoadContext();
+  ReferrerPolicy referrerPolicy;
+
+  if (aMaybeActiveScript) {
+    // https://html.spec.whatwg.org/multipage/webappapis.html#hostloadimportedmodule
+    // Step 6.3. Set fetchOptions to the new descendant script fetch options for
+    // referencingScript's fetch options.
+    options = aMaybeActiveScript->GetFetchOptions();
+    referrerPolicy = aMaybeActiveScript->ReferrerPolicy();
+    baseURL = aMaybeActiveScript->BaseURL();
+  } else {
+    // We don't have a referencing script so fall back on using
+    // options from the document. This can happen when the user
+    // triggers an inline event handler, as there is no active script
+    // there.
+    Document* document = GetScriptLoader()->GetDocument();
+
+    nsCOMPtr<nsIPrincipal> principal = GetGlobalObject()->PrincipalOrNull();
+    MOZ_ASSERT_IF(GetKind() == WebExtension,
+                  BasePrincipal::Cast(principal)->ContentScriptAddonPolicy());
+    MOZ_ASSERT_IF(GetKind() == Normal, principal == document->NodePrincipal());
+
+    // https://html.spec.whatwg.org/multipage/webappapis.html#hostloadimportedmodule
+    // Step 4. Let fetchOptions be the default classic script fetch options.
+    //
+    // https://html.spec.whatwg.org/multipage/webappapis.html#default-classic-script-fetch-options
+    // The default classic script fetch options are a script fetch options whose
+    // cryptographic nonce is the empty string, integrity metadata is the empty
+    // string, parser metadata is "not-parser-inserted", credentials mode is
+    // "same-origin", referrer policy is the empty string, and fetch priority is
+    // "auto".
+    options = new ScriptFetchOptions(
+        mozilla::CORS_NONE, /* aNonce = */ u""_ns, RequestPriority::Auto,
+        ParserMetadata::NotParserInserted, principal, nullptr);
+    referrerPolicy = document->GetReferrerPolicy();
+    baseURL = document->GetDocBaseURI();
+  }
+
+  context->mIsInline = false;
+  context->mScriptMode = ScriptLoadContext::ScriptMode::eAsync;
+
+  RefPtr<ModuleLoadRequest> request = new ModuleLoadRequest(
+      aURI, referrerPolicy, options, SRIMetadata(), baseURL, context, true,
+      /* is top level */ true, /* is dynamic import */
+      this, ModuleLoadRequest::NewVisitedSetForTopLevelImport(aURI), nullptr);
+
+  request->SetDynamicImport(aMaybeActiveScript, aSpecifier, aPromise);
+  request->NoCacheEntryFound();
+
+  return request.forget();
+}
+
+ModuleLoader::~ModuleLoader() {
+  LOG(("ModuleLoader::~ModuleLoader %p", this));
+  mLoader = nullptr;
+}
+
+#undef LOG
+#undef LOG_ENABLED
+
+}  // namespace mozilla::dom
diff --git a/dom/script/ModuleLoader.h b/dom/script/ModuleLoader.h
new file mode 100644
index 0000000000..4f3f270f0b
--- /dev/null
+++ b/dom/script/ModuleLoader.h
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ModuleLoader_h
+#define mozilla_dom_ModuleLoader_h
+
+#include "mozilla/dom/ScriptLoadContext.h"
+#include "js/loader/ModuleLoaderBase.h"
+#include "js/loader/ScriptLoadRequest.h"
+#include "ScriptLoader.h"
+
+class nsIURI;
+
+namespace JS {
+
+class CompileOptions;
+
+namespace loader {
+
+class ModuleLoadRequest;
+
+}  // namespace loader
+}  // namespace JS
+
+namespace mozilla::dom {
+
+class ScriptLoader;
+class SRIMetadata;
+
+//////////////////////////////////////////////////////////////
+// DOM Module loader implementation
+//////////////////////////////////////////////////////////////
+
+class ModuleLoader final : public JS::loader::ModuleLoaderBase {
+ private:
+  virtual ~ModuleLoader();
+
+ public:
+  enum Kind { Normal, WebExtension };
+
+  ModuleLoader(ScriptLoader* aLoader, nsIGlobalObject* aGlobalObject,
+               Kind aKind);
+
+  Kind GetKind() const { return mKind; }
+
+  ScriptLoader* GetScriptLoader();
+
+  bool CanStartLoad(ModuleLoadRequest* aRequest, nsresult* aRvOut) override;
+
+  nsresult StartFetch(ModuleLoadRequest* aRequest) override;
+
+  void OnModuleLoadComplete(ModuleLoadRequest* aRequest) override;
+
+  nsresult CompileFetchedModule(
+      JSContext* aCx, JS::Handle<JSObject*> aGlobal,
+      JS::CompileOptions& aOptions, ModuleLoadRequest* aRequest,
+      JS::MutableHandle<JSObject*> aModuleScript) override;
+
+  // Create a top-level module load request.
+  static already_AddRefed<ModuleLoadRequest> CreateTopLevel(
+      nsIURI* aURI, ReferrerPolicy aReferrerPolicy,
+      ScriptFetchOptions* aFetchOptions, const SRIMetadata& aIntegrity,
+      nsIURI* aReferrer, ScriptLoader* aLoader, ScriptLoadContext* aContext);
+
+  // Create a module load request for a static module import.
+  already_AddRefed<ModuleLoadRequest> CreateStaticImport(
+      nsIURI* aURI, ModuleLoadRequest* aParent) override;
+
+  // Create a module load request for a dynamic module import.
+  already_AddRefed<ModuleLoadRequest> CreateDynamicImport(
+      JSContext* aCx, nsIURI* aURI, LoadedScript* aMaybeActiveScript,
+      JS::Handle<JSString*> aSpecifier,
+      JS::Handle<JSObject*> aPromise) override;
+
+  static ModuleLoader* From(ModuleLoaderBase* aLoader) {
+    return static_cast<ModuleLoader*>(aLoader);
+  }
+
+  void AsyncExecuteInlineModule(ModuleLoadRequest* aRequest);
+  void ExecuteInlineModule(ModuleLoadRequest* aRequest);
+
+ private:
+  const Kind mKind;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_ModuleLoader_h
diff --git a/dom/script/ScriptCompression.cpp b/dom/script/ScriptCompression.cpp
new file mode 100644
index 0000000000..837fefe096
--- /dev/null
+++ b/dom/script/ScriptCompression.cpp
@@ -0,0 +1,176 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "zlib.h"
+#include "ScriptLoadRequest.h"
+#include "ScriptLoader.h"
+#include "mozilla/PerfStats.h"
+#include "mozilla/ProfilerMarkers.h"
+#include "mozilla/Vector.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/StaticPrefs_browser.h"
+
+using namespace mozilla;
+
+namespace JS::loader {
+
+#undef LOG
+#define LOG(args)                                       \
+  MOZ_LOG(mozilla::dom::ScriptLoader::gScriptLoaderLog, \
+          mozilla::LogLevel::Debug, args)
+
+/*
+ * ScriptBytecodeDataLayout
+ *
+ * ScriptBytecodeDataLayout provides accessors to maintain the correct data
+ * layout of the ScriptLoadRequest's script bytecode buffer.
+ */
+class ScriptBytecodeDataLayout {
+ public:
+  explicit ScriptBytecodeDataLayout(mozilla::Vector<uint8_t>& aBytecode,
+                                    size_t aBytecodeOffset)
+      : mBytecode(aBytecode), mBytecodeOffset(aBytecodeOffset) {}
+
+  uint8_t* prelude() const { return mBytecode.begin(); }
+  size_t preludeLength() const { return mBytecodeOffset; }
+
+  uint8_t* bytecode() const { return prelude() + mBytecodeOffset; }
+  size_t bytecodeLength() const { return mBytecode.length() - preludeLength(); }
+
+  mozilla::Vector<uint8_t>& mBytecode;
+  size_t mBytecodeOffset;
+};
+
+/*
+ * ScriptBytecodeCompressedDataLayout
+ *
+ * ScriptBytecodeCompressedDataLayout provides accessors to maintain the correct
+ * data layout of a compressed script bytecode buffer.
+ */
+class ScriptBytecodeCompressedDataLayout {
+ public:
+  using UncompressedLengthType = uint32_t;
+
+  explicit ScriptBytecodeCompressedDataLayout(
+      mozilla::Vector<uint8_t>& aBytecode, size_t aBytecodeOffset)
+      : mBytecode(aBytecode), mBytecodeOffset(aBytecodeOffset) {}
+
+  uint8_t* prelude() const { return mBytecode.begin(); }
+  size_t preludeLength() const { return mBytecodeOffset; }
+
+  uint8_t* uncompressedLength() const { return prelude() + mBytecodeOffset; }
+  size_t uncompressedLengthLength() const {
+    return sizeof(UncompressedLengthType);
+  }
+
+  uint8_t* bytecode() const {
+    return uncompressedLength() + uncompressedLengthLength();
+  }
+  size_t bytecodeLength() const {
+    return mBytecode.length() - uncompressedLengthLength() - preludeLength();
+  }
+
+  mozilla::Vector<uint8_t>& mBytecode;
+  size_t mBytecodeOffset;
+};
+
+bool ScriptBytecodeCompress(Vector<uint8_t>& aBytecodeBuf,
+                            size_t aBytecodeOffset,
+                            Vector<uint8_t>& aCompressedBytecodeBufOut) {
+  // TODO probably need to move this to a helper thread
+
+  AUTO_PROFILER_MARKER_TEXT("ScriptBytecodeCompress", JS, {}, ""_ns);
+  PerfStats::AutoMetricRecording<PerfStats::Metric::JSBC_Compression>
+      autoRecording;
+
+  ScriptBytecodeDataLayout uncompressedLayout(aBytecodeBuf, aBytecodeOffset);
+  ScriptBytecodeCompressedDataLayout compressedLayout(
+      aCompressedBytecodeBufOut, uncompressedLayout.preludeLength());
+  ScriptBytecodeCompressedDataLayout::UncompressedLengthType
+      uncompressedLength = uncompressedLayout.bytecodeLength();
+  z_stream zstream{.next_in = uncompressedLayout.bytecode(),
+                   .avail_in = uncompressedLength};
+
+  // Note: deflateInit needs to be called before deflateBound.
+  const uint32_t compressionLevel =
+      StaticPrefs::browser_cache_jsbc_compression_level();
+  if (deflateInit(&zstream, compressionLevel) != Z_OK) {
+    LOG(
+        ("ScriptLoadRequest: Unable to initialize bytecode cache "
+         "compression."));
+    return false;
+  }
+  auto autoDestroy = MakeScopeExit([&]() { deflateEnd(&zstream); });
+
+  auto compressedLength = deflateBound(&zstream, uncompressedLength);
+  if (!aCompressedBytecodeBufOut.resizeUninitialized(
+          compressedLength + compressedLayout.preludeLength() +
+          compressedLayout.uncompressedLengthLength())) {
+    return false;
+  }
+  memcpy(compressedLayout.prelude(), uncompressedLayout.prelude(),
+         uncompressedLayout.preludeLength());
+  memcpy(compressedLayout.uncompressedLength(), &uncompressedLength,
+         sizeof(uncompressedLength));
+  zstream.next_out = compressedLayout.bytecode();
+  zstream.avail_out = compressedLength;
+
+  int ret = deflate(&zstream, Z_FINISH);
+  if (ret == Z_MEM_ERROR) {
+    return false;
+  }
+  MOZ_RELEASE_ASSERT(ret == Z_STREAM_END);
+
+  aCompressedBytecodeBufOut.shrinkTo(zstream.next_out -
+                                     aCompressedBytecodeBufOut.begin());
+  return true;
+}
+
+bool ScriptBytecodeDecompress(Vector<uint8_t>& aCompressedBytecodeBuf,
+                              size_t aBytecodeOffset,
+                              Vector<uint8_t>& aBytecodeBufOut) {
+  AUTO_PROFILER_MARKER_TEXT("ScriptBytecodeDecompress", JS, {}, ""_ns);
+  PerfStats::AutoMetricRecording<PerfStats::Metric::JSBC_Decompression>
+      autoRecording;
+
+  ScriptBytecodeDataLayout uncompressedLayout(aBytecodeBufOut, aBytecodeOffset);
+  ScriptBytecodeCompressedDataLayout compressedLayout(
+      aCompressedBytecodeBuf, uncompressedLayout.preludeLength());
+  ScriptBytecodeCompressedDataLayout::UncompressedLengthType uncompressedLength;
+  memcpy(&uncompressedLength, compressedLayout.uncompressedLength(),
+         compressedLayout.uncompressedLengthLength());
+  if (!aBytecodeBufOut.resizeUninitialized(uncompressedLayout.preludeLength() +
+                                           uncompressedLength)) {
+    return false;
+  }
+  memcpy(uncompressedLayout.prelude(), compressedLayout.prelude(),
+         compressedLayout.preludeLength());
+
+  z_stream zstream{nullptr};
+  zstream.next_in = compressedLayout.bytecode();
+  zstream.avail_in = static_cast<uint32_t>(compressedLayout.bytecodeLength());
+  zstream.next_out = uncompressedLayout.bytecode();
+  zstream.avail_out = uncompressedLength;
+  if (inflateInit(&zstream) != Z_OK) {
+    LOG(("ScriptLoadRequest: inflateInit FAILED (%s)", zstream.msg));
+    return false;
+  }
+  auto autoDestroy = MakeScopeExit([&]() { inflateEnd(&zstream); });
+
+  int ret = inflate(&zstream, Z_NO_FLUSH);
+  bool ok = (ret == Z_OK || ret == Z_STREAM_END) && zstream.avail_in == 0;
+  if (!ok) {
+    LOG(("ScriptLoadReques: inflate FAILED (%s)", zstream.msg));
+    return false;
+  }
+
+  return true;
+}
+
+#undef LOG
+
+}  // namespace JS::loader
diff --git a/dom/script/ScriptCompression.h b/dom/script/ScriptCompression.h
new file mode 100644
index 0000000000..c824aa9c15
--- /dev/null
+++ b/dom/script/ScriptCompression.h
@@ -0,0 +1,43 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef js_loader_ScriptCompression_h
+#define js_loader_ScriptCompression_h
+
+#include "ErrorList.h"
+#include "mozilla/Vector.h"
+
+namespace JS::loader {
+
+class ScriptLoadRequest;
+
+/**
+ * Compress the bytecode stored in a buffer. All data before the bytecode is
+ * copied into the output buffer without modification.
+ *
+ * @param aBytecodeBuf buffer containing the uncompressed bytecode
+ * @param aBytecodeOffset offset of the bytecode in the buffer
+ * @param aCompressedBytecodeBufOut buffer to store the compressed bytecode in
+ */
+bool ScriptBytecodeCompress(
+    mozilla::Vector<uint8_t>& aBytecodeBuf, size_t aBytecodeOffset,
+    mozilla::Vector<uint8_t>& aCompressedBytecodeBufOut);
+
+/**
+ * Uncompress the bytecode stored in a buffer. All data before the bytecode is
+ * copied into the output buffer without modification.
+ *
+ * @param aCompressedBytecodeBuf buffer containing the compressed bytecode
+ * @param aBytecodeOffset offset of the bytecode in the buffer
+ * @param aBytecodeBufOut buffer to store the uncompressed bytecode in
+ */
+bool ScriptBytecodeDecompress(mozilla::Vector<uint8_t>& aCompressedBytecodeBuf,
+                              size_t aBytecodeOffset,
+                              mozilla::Vector<uint8_t>& aBytecodeBufOut);
+
+}  // namespace JS::loader
+
+#endif  // js_loader_ScriptCompression_h
diff --git a/dom/script/ScriptDecoding.h b/dom/script/ScriptDecoding.h
new file mode 100644
index 0000000000..ea4c4aeef3
--- /dev/null
+++ b/dom/script/ScriptDecoding.h
@@ -0,0 +1,88 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Script decoding templates for processing byte data as UTF-8 or UTF-16. */
+
+#ifndef mozilla_dom_ScriptDecoding_h
+#define mozilla_dom_ScriptDecoding_h
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT
+#include "mozilla/CheckedInt.h"  // mozilla::CheckedInt
+#include "mozilla/Encoding.h"    // mozilla::Decoder
+#include "mozilla/Span.h"        // mozilla::Span
+#include "mozilla/UniquePtr.h"   // mozilla::UniquePtr
+
+#include <stddef.h>     // size_t
+#include <stdint.h>     // uint8_t, uint32_t
+#include <type_traits>  // std::is_same
+
+namespace mozilla::dom {
+
+template <typename Unit>
+struct ScriptDecoding {
+  static_assert(std::is_same<Unit, char16_t>::value ||
+                    std::is_same<Unit, Utf8Unit>::value,
+                "must be either UTF-8 or UTF-16");
+};
+
+template <>
+struct ScriptDecoding<char16_t> {
+  static CheckedInt<size_t> MaxBufferLength(const UniquePtr<Decoder>& aDecoder,
+                                            uint32_t aByteLength) {
+    return aDecoder->MaxUTF16BufferLength(aByteLength);
+  }
+
+  static size_t DecodeInto(const UniquePtr<Decoder>& aDecoder,
+                           const Span<const uint8_t>& aSrc,
+                           Span<char16_t> aDest, bool aEndOfSource) {
+    uint32_t result;
+    size_t read;
+    size_t written;
+    std::tie(result, read, written, std::ignore) =
+        aDecoder->DecodeToUTF16(aSrc, aDest, aEndOfSource);
+    MOZ_ASSERT(result == kInputEmpty);
+    MOZ_ASSERT(read == aSrc.Length());
+    MOZ_ASSERT(written <= aDest.Length());
+
+    return written;
+  }
+};
+
+template <>
+struct ScriptDecoding<Utf8Unit> {
+  static CheckedInt<size_t> MaxBufferLength(const UniquePtr<Decoder>& aDecoder,
+                                            uint32_t aByteLength) {
+    return aDecoder->MaxUTF8BufferLength(aByteLength);
+  }
+
+  static size_t DecodeInto(const UniquePtr<Decoder>& aDecoder,
+                           const Span<const uint8_t>& aSrc,
+                           Span<Utf8Unit> aDest, bool aEndOfSource) {
+    uint32_t result;
+    size_t read;
+    size_t written;
+    // Until C++ char8_t happens, our decoder APIs deal in |uint8_t| while
+    // |Utf8Unit| internally deals with |char|, so there's inevitable impedance
+    // mismatch between |aDest| as |Utf8Unit| and |AsWritableBytes(aDest)| as
+    // |Span<uint8_t>|.  :-(  The written memory will be interpreted through
+    // |char Utf8Unit::mValue| which is *permissible* because any object's
+    // memory can be interpreted as |char|.  Unfortunately, until
+    // twos-complement is mandated, we have to play fast and loose and *hope*
+    // interpreting memory storing |uint8_t| as |char| will pick up the desired
+    // wrapped-around value.  ¯\_(ツ)_/¯
+    std::tie(result, read, written, std::ignore) =
+        aDecoder->DecodeToUTF8(aSrc, AsWritableBytes(aDest), aEndOfSource);
+    MOZ_ASSERT(result == kInputEmpty);
+    MOZ_ASSERT(read == aSrc.Length());
+    MOZ_ASSERT(written <= aDest.Length());
+
+    return written;
+  }
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_ScriptDecoding_h
diff --git a/dom/script/ScriptElement.cpp b/dom/script/ScriptElement.cpp
new file mode 100644
index 0000000000..1887aa5937
--- /dev/null
+++ b/dom/script/ScriptElement.cpp
@@ -0,0 +1,222 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ScriptElement.h"
+#include "ScriptLoader.h"
+#include "mozilla/BasicEvents.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/EventDispatcher.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/MutationEventBinding.h"
+#include "nsContentUtils.h"
+#include "nsThreadUtils.h"
+#include "nsPresContext.h"
+#include "nsIParser.h"
+#include "nsGkAtoms.h"
+#include "nsContentSink.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+NS_IMETHODIMP
+ScriptElement::ScriptAvailable(nsresult aResult, nsIScriptElement* aElement,
+                               bool aIsInlineClassicScript, nsIURI* aURI,
+                               uint32_t aLineNo) {
+  if (!aIsInlineClassicScript && NS_FAILED(aResult)) {
+    nsCOMPtr<nsIParser> parser = do_QueryReferent(mCreatorParser);
+    if (parser) {
+      nsCOMPtr<nsIContentSink> sink = parser->GetContentSink();
+      if (sink) {
+        nsCOMPtr<Document> parserDoc = do_QueryInterface(sink->GetTarget());
+        if (GetAsContent()->OwnerDoc() != parserDoc) {
+          // Suppress errors when we've moved between docs.
+          // /html/semantics/scripting-1/the-script-element/moving-between-documents/move-back-iframe-fetch-error-external-module.html
+          // See also https://bugzilla.mozilla.org/show_bug.cgi?id=1849107
+          return NS_OK;
+        }
+      }
+    }
+
+    if (parser) {
+      parser->IncrementScriptNestingLevel();
+    }
+    nsresult rv = FireErrorEvent();
+    if (parser) {
+      parser->DecrementScriptNestingLevel();
+    }
+    return rv;
+  }
+  return NS_OK;
+}
+
+/* virtual */
+nsresult ScriptElement::FireErrorEvent() {
+  nsIContent* cont = GetAsContent();
+
+  return nsContentUtils::DispatchTrustedEvent(
+      cont->OwnerDoc(), cont, u"error"_ns, CanBubble::eNo, Cancelable::eNo);
+}
+
+NS_IMETHODIMP
+ScriptElement::ScriptEvaluated(nsresult aResult, nsIScriptElement* aElement,
+                               bool aIsInline) {
+  nsresult rv = NS_OK;
+  if (!aIsInline) {
+    nsCOMPtr<nsIContent> cont = GetAsContent();
+
+    RefPtr<nsPresContext> presContext =
+        nsContentUtils::GetContextForContent(cont);
+
+    nsEventStatus status = nsEventStatus_eIgnore;
+    EventMessage message = NS_SUCCEEDED(aResult) ? eLoad : eLoadError;
+    WidgetEvent event(true, message);
+    // Load event doesn't bubble.
+    event.mFlags.mBubbles = (message != eLoad);
+
+    EventDispatcher::Dispatch(cont, presContext, &event, nullptr, &status);
+  }
+
+  return rv;
+}
+
+void ScriptElement::CharacterDataChanged(nsIContent* aContent,
+                                         const CharacterDataChangeInfo&) {
+  MaybeProcessScript();
+}
+
+void ScriptElement::AttributeChanged(Element* aElement, int32_t aNameSpaceID,
+                                     nsAtom* aAttribute, int32_t aModType,
+                                     const nsAttrValue* aOldValue) {
+  // https://html.spec.whatwg.org/#script-processing-model
+  // When a script element el that is not parser-inserted experiences one of the
+  // events listed in the following list, the user agent must immediately
+  // prepare the script element el:
+  //  - The script element is connected and has a src attribute set where
+  //  previously the element had no such attribute.
+  if (aElement->IsSVGElement() && ((aNameSpaceID != kNameSpaceID_XLink &&
+                                    aNameSpaceID != kNameSpaceID_None) ||
+                                   aAttribute != nsGkAtoms::href)) {
+    return;
+  }
+  if (aElement->IsHTMLElement() &&
+      (aNameSpaceID != kNameSpaceID_None || aAttribute != nsGkAtoms::src)) {
+    return;
+  }
+  if (mParserCreated == NOT_FROM_PARSER &&
+      aModType == MutationEvent_Binding::ADDITION) {
+    auto* cont = GetAsContent();
+    if (cont->IsInComposedDoc()) {
+      MaybeProcessScript();
+    }
+  }
+}
+
+void ScriptElement::ContentAppended(nsIContent* aFirstNewContent) {
+  MaybeProcessScript();
+}
+
+void ScriptElement::ContentInserted(nsIContent* aChild) {
+  MaybeProcessScript();
+}
+
+bool ScriptElement::MaybeProcessScript() {
+  nsIContent* cont = GetAsContent();
+
+  NS_ASSERTION(cont->DebugGetSlots()->mMutationObservers.contains(this),
+               "You forgot to add self as observer");
+
+  // https://html.spec.whatwg.org/#parsing-main-incdata
+  // An end tag whose tag name is "script"
+  //  - If the active speculative HTML parser is null and the JavaScript
+  // execution context stack is empty, then perform a microtask checkpoint.
+  nsContentUtils::AddScriptRunner(NS_NewRunnableFunction(
+      "ScriptElement::MaybeProcessScript", []() { nsAutoMicroTask mt; }));
+
+  if (mAlreadyStarted || !mDoneAddingChildren || !cont->GetComposedDoc() ||
+      mMalformed) {
+    return false;
+  }
+
+  if (!HasScriptContent()) {
+    // In the case of an empty, non-external classic script, there is nothing
+    // to process. However, we must perform a microtask checkpoint afterwards,
+    // as per https://html.spec.whatwg.org/#clean-up-after-running-script
+    if (mKind == JS::loader::ScriptKind::eClassic && !mExternal) {
+      nsContentUtils::AddScriptRunner(NS_NewRunnableFunction(
+          "ScriptElement::MaybeProcessScript", []() { nsAutoMicroTask mt; }));
+    }
+    return false;
+  }
+
+  // Check the type attribute to determine language and version. If type exists,
+  // it trumps the deprecated 'language='.
+  nsAutoString type;
+  bool hasType = GetScriptType(type);
+  if (!type.IsEmpty()) {
+    NS_ENSURE_TRUE(nsContentUtils::IsJavascriptMIMEType(type) ||
+                       type.LowerCaseEqualsASCII("module") ||
+                       type.LowerCaseEqualsASCII("importmap"),
+                   false);
+  } else if (!hasType) {
+    // "language" is a deprecated attribute of HTML, so we check it only for
+    // HTML script elements.
+    if (cont->IsHTMLElement()) {
+      nsAutoString language;
+      cont->AsElement()->GetAttr(nsGkAtoms::language, language);
+      if (!language.IsEmpty() &&
+          !nsContentUtils::IsJavaScriptLanguage(language)) {
+        return false;
+      }
+    }
+  }
+
+  Document* ownerDoc = cont->OwnerDoc();
+  FreezeExecutionAttrs(ownerDoc);
+
+  mAlreadyStarted = true;
+
+  nsCOMPtr<nsIParser> parser = ((nsIScriptElement*)this)->GetCreatorParser();
+  if (parser) {
+    nsCOMPtr<nsIContentSink> sink = parser->GetContentSink();
+    if (sink) {
+      nsCOMPtr<Document> parserDoc = do_QueryInterface(sink->GetTarget());
+      if (ownerDoc != parserDoc) {
+        // Refactor this: https://bugzilla.mozilla.org/show_bug.cgi?id=1849107
+        return false;
+      }
+    }
+  }
+
+  RefPtr<ScriptLoader> loader = ownerDoc->ScriptLoader();
+  return loader->ProcessScriptElement(this);
+}
+
+bool ScriptElement::GetScriptType(nsAString& aType) {
+  Element* element = GetAsContent()->AsElement();
+
+  nsAutoString type;
+  if (!element->GetAttr(nsGkAtoms::type, type)) {
+    return false;
+  }
+
+  // ASCII whitespace https://infra.spec.whatwg.org/#ascii-whitespace:
+  // U+0009 TAB, U+000A LF, U+000C FF, U+000D CR, or U+0020 SPACE.
+  static const char kASCIIWhitespace[] = "\t\n\f\r ";
+
+  const bool wasEmptyBeforeTrim = type.IsEmpty();
+  type.Trim(kASCIIWhitespace);
+
+  // If the value before trim was not empty and the value is now empty, do not
+  // trim as we want to retain pure whitespace (by restoring original value)
+  // because we need to treat "" and " " (etc) differently.
+  if (!wasEmptyBeforeTrim && type.IsEmpty()) {
+    return element->GetAttr(nsGkAtoms::type, aType);
+  }
+
+  aType.Assign(type);
+  return true;
+}
diff --git a/dom/script/ScriptElement.h b/dom/script/ScriptElement.h
new file mode 100644
index 0000000000..568ed9f786
--- /dev/null
+++ b/dom/script/ScriptElement.h
@@ -0,0 +1,54 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ScriptElement_h
+#define mozilla_dom_ScriptElement_h
+
+#include "mozilla/Attributes.h"
+#include "nsIScriptLoaderObserver.h"
+#include "nsIScriptElement.h"
+#include "nsStubMutationObserver.h"
+
+namespace mozilla::dom {
+
+/**
+ * Baseclass useful for script elements (such as <xhtml:script> and
+ * <svg:script>). Currently the class assumes that only the 'src'
+ * attribute and the children of the class affect what script to execute.
+ */
+
+class ScriptElement : public nsIScriptElement, public nsStubMutationObserver {
+ public:
+  // nsIScriptLoaderObserver
+  NS_DECL_NSISCRIPTLOADEROBSERVER
+
+  // nsIMutationObserver
+  NS_DECL_NSIMUTATIONOBSERVER_CHARACTERDATACHANGED
+  NS_DECL_NSIMUTATIONOBSERVER_ATTRIBUTECHANGED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTAPPENDED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTINSERTED
+
+  explicit ScriptElement(FromParser aFromParser)
+      : nsIScriptElement(aFromParser) {}
+
+  virtual nsresult FireErrorEvent() override;
+
+  virtual bool GetScriptType(nsAString& aType) override;
+
+ protected:
+  // Internal methods
+
+  /**
+   * Check if this element contains any script, linked or inline
+   */
+  virtual bool HasScriptContent() = 0;
+
+  virtual bool MaybeProcessScript() override;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_ScriptElement_h
diff --git a/dom/script/ScriptLoadContext.cpp b/dom/script/ScriptLoadContext.cpp
new file mode 100644
index 0000000000..75d54a309d
--- /dev/null
+++ b/dom/script/ScriptLoadContext.cpp
@@ -0,0 +1,206 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "GeckoProfiler.h"
+
+#include "mozilla/dom/Document.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/Unused.h"
+#include "mozilla/Utf8.h"  // mozilla::Utf8Unit
+
+#include "js/SourceText.h"
+#include "js/loader/LoadContextBase.h"
+#include "js/loader/ModuleLoadRequest.h"
+
+#include "ScriptLoadContext.h"
+#include "ModuleLoadRequest.h"
+#include "nsContentUtils.h"
+#include "nsICacheInfoChannel.h"
+#include "nsIClassOfService.h"
+#include "nsISupportsPriority.h"
+
+namespace mozilla::dom {
+
+//////////////////////////////////////////////////////////////
+// ScriptLoadContext
+//////////////////////////////////////////////////////////////
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ScriptLoadContext)
+NS_INTERFACE_MAP_END_INHERITING(JS::loader::LoadContextBase)
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(ScriptLoadContext)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(ScriptLoadContext,
+                                                JS::loader::LoadContextBase)
+  MOZ_ASSERT(!tmp->mCompileOrDecodeTask);
+  tmp->MaybeUnblockOnload();
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(ScriptLoadContext,
+                                                  JS::loader::LoadContextBase)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mLoadBlockedDocument)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_ADDREF_INHERITED(ScriptLoadContext, JS::loader::LoadContextBase)
+NS_IMPL_RELEASE_INHERITED(ScriptLoadContext, JS::loader::LoadContextBase)
+
+ScriptLoadContext::ScriptLoadContext()
+    : JS::loader::LoadContextBase(JS::loader::ContextKind::Window),
+      mScriptMode(ScriptMode::eBlocking),
+      mScriptFromHead(false),
+      mIsInline(true),
+      mInDeferList(false),
+      mInAsyncList(false),
+      mIsNonAsyncScriptInserted(false),
+      mIsXSLT(false),
+      mInCompilingList(false),
+      mIsTracking(false),
+      mWasCompiledOMT(false),
+      mLineNo(1),
+      mColumnNo(0),
+      mIsPreload(false),
+      mUnreportedPreloadError(NS_OK) {}
+
+ScriptLoadContext::~ScriptLoadContext() {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  // Off-thread parsing must have completed or cancelled by this point.
+  MOZ_DIAGNOSTIC_ASSERT(!mCompileOrDecodeTask);
+
+  mRequest = nullptr;
+
+  MaybeUnblockOnload();
+}
+
+void ScriptLoadContext::BlockOnload(Document* aDocument) {
+  MOZ_ASSERT(!mLoadBlockedDocument);
+  aDocument->BlockOnload();
+  mLoadBlockedDocument = aDocument;
+}
+
+void ScriptLoadContext::MaybeUnblockOnload() {
+  if (mLoadBlockedDocument) {
+    mLoadBlockedDocument->UnblockOnload(false);
+    mLoadBlockedDocument = nullptr;
+  }
+}
+
+void ScriptLoadContext::MaybeCancelOffThreadScript() {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  if (!mCompileOrDecodeTask) {
+    return;
+  }
+
+  // Cancel the task if it hasn't been started yet or wait for it to finish.
+  mCompileOrDecodeTask->Cancel();
+  mCompileOrDecodeTask = nullptr;
+
+  MaybeUnblockOnload();
+}
+
+void ScriptLoadContext::SetScriptMode(bool aDeferAttr, bool aAsyncAttr,
+                                      bool aLinkPreload) {
+  if (aLinkPreload) {
+    mScriptMode = ScriptMode::eLinkPreload;
+  } else if (aAsyncAttr) {
+    mScriptMode = ScriptMode::eAsync;
+  } else if (aDeferAttr || mRequest->IsModuleRequest()) {
+    mScriptMode = ScriptMode::eDeferred;
+  } else {
+    mScriptMode = ScriptMode::eBlocking;
+  }
+}
+
+// static
+void ScriptLoadContext::PrioritizeAsPreload(nsIChannel* aChannel) {
+  if (nsCOMPtr<nsIClassOfService> cos = do_QueryInterface(aChannel)) {
+    cos->AddClassFlags(nsIClassOfService::Unblocked);
+  }
+  if (nsCOMPtr<nsISupportsPriority> sp = do_QueryInterface(aChannel)) {
+    sp->AdjustPriority(nsISupportsPriority::PRIORITY_HIGHEST);
+  }
+}
+
+bool ScriptLoadContext::IsPreload() const {
+  if (mRequest->IsModuleRequest() && !mRequest->IsTopLevel()) {
+    JS::loader::ModuleLoadRequest* root =
+        mRequest->AsModuleRequest()->GetRootModule();
+    return root->GetScriptLoadContext()->IsPreload();
+  }
+
+  MOZ_ASSERT_IF(mIsPreload, !GetScriptElement());
+  return mIsPreload;
+}
+
+bool ScriptLoadContext::CompileStarted() const {
+  return mRequest->IsCompiling() || (mRequest->IsFinished() && mWasCompiledOMT);
+}
+
+nsIScriptElement* ScriptLoadContext::GetScriptElement() const {
+  nsCOMPtr<nsIScriptElement> scriptElement =
+      do_QueryInterface(mRequest->mFetchOptions->mElement);
+  return scriptElement;
+}
+
+void ScriptLoadContext::SetIsLoadRequest(nsIScriptElement* aElement) {
+  MOZ_ASSERT(aElement);
+  MOZ_ASSERT(!GetScriptElement());
+  MOZ_ASSERT(IsPreload());
+  // We are not tracking our own element, and are relying on the one in
+  // FetchOptions.
+  mRequest->mFetchOptions->mElement = do_QueryInterface(aElement);
+  mIsPreload = false;
+}
+
+void ScriptLoadContext::GetProfilerLabel(nsACString& aOutString) {
+  if (!profiler_is_active()) {
+    aOutString.Append("<script> element");
+    return;
+  }
+  aOutString.Append("<script");
+  if (IsAsyncScript()) {
+    aOutString.Append(" async");
+  } else if (IsDeferredScript()) {
+    aOutString.Append(" defer");
+  }
+  if (mRequest->IsModuleRequest()) {
+    aOutString.Append(" type=\"module\"");
+  }
+
+  nsAutoCString url;
+  if (mRequest->mURI) {
+    mRequest->mURI->GetAsciiSpec(url);
+  } else {
+    url = "<unknown>";
+  }
+
+  if (mIsInline) {
+    if (GetParserCreated() != NOT_FROM_PARSER) {
+      aOutString.Append("> inline at line ");
+      aOutString.AppendInt(mLineNo);
+      aOutString.Append(" of ");
+    } else {
+      aOutString.Append("> inline (dynamically created) in ");
+    }
+    aOutString.Append(url);
+  } else {
+    aOutString.Append(" src=\"");
+    aOutString.Append(url);
+    aOutString.Append("\">");
+  }
+}
+
+already_AddRefed<JS::Stencil> ScriptLoadContext::StealOffThreadResult(
+    JSContext* aCx, JS::InstantiationStorage* aInstantiationStorage) {
+  RefPtr<CompileOrDecodeTask> compileOrDecodeTask =
+      mCompileOrDecodeTask.forget();
+
+  return compileOrDecodeTask->StealResult(aCx, aInstantiationStorage);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/script/ScriptLoadContext.h b/dom/script/ScriptLoadContext.h
new file mode 100644
index 0000000000..74f7c6fe4f
--- /dev/null
+++ b/dom/script/ScriptLoadContext.h
@@ -0,0 +1,253 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ScriptLoadContext_h
+#define mozilla_dom_ScriptLoadContext_h
+
+#include "js/AllocPolicy.h"
+#include "js/ColumnNumber.h"    // JS::ColumnNumberOneOrigin
+#include "js/CompileOptions.h"  // JS::OwningCompileOptions
+#include "js/experimental/JSStencil.h"  // JS::FrontendContext, JS::Stencil, JS::InstantiationStorage
+#include "js/RootingAPI.h"
+#include "js/SourceText.h"
+#include "js/Transcoding.h"  // JS::TranscodeResult
+#include "js/TypeDecls.h"
+#include "js/loader/LoadContextBase.h"
+#include "js/loader/ScriptKind.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/CORSMode.h"
+#include "mozilla/dom/SRIMetadata.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MaybeOneOf.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/PreloaderBase.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/TaskController.h"  // mozilla::Task
+#include "mozilla/Utf8.h"            // mozilla::Utf8Unit
+#include "mozilla/Variant.h"
+#include "mozilla/Vector.h"
+#include "nsCOMPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIScriptElement.h"
+
+class nsICacheInfoChannel;
+struct JSContext;
+
+namespace mozilla::dom {
+
+class Element;
+
+/*
+ * DOM specific ScriptLoadContext.
+ *
+ * ScriptLoadContexts augment the loading of a ScriptLoadRequest. They
+ * describe how a ScriptLoadRequests loading and evaluation needs to be
+ * augmented, based on the information provided by the loading context. In
+ * the case of the DOM, the ScriptLoadContext is used to identify how a script
+ * should be loaded according to information found in the HTML document into
+ * which it will be loaded. The following fields describe how the
+ * ScriptLoadRequest will be loaded.
+ *
+ *    * mScriptMode
+ *        stores the mode (Async, Sync, Deferred), and preload, which
+ *        allows the ScriptLoader to decide if the script should be pushed
+ *        offThread, or if the preloaded request should be used.
+ *    * mScriptFromHead
+ *        Set when the script tag is in the head, and should be treated as
+ *        a blocking script
+ *    * mIsInline
+ *        Set for scripts whose bodies are inline in the html. In this case,
+ *        the script does not need to be fetched first.
+ *    * mIsXSLT
+ *        Set if we are in an XSLT request.
+ *    * mIsPreload
+ *        Set for scripts that are preloaded in a
+ *        <link rel="preload" as="script"> or <link rel="modulepreload">
+ *        element.
+ *
+ * In addition to describing how the ScriptLoadRequest will be loaded by the
+ * DOM ScriptLoader, the ScriptLoadContext contains fields that facilitate
+ * those custom behaviors, including support for offthread parsing and preload
+ * element specific controls.
+ *
+ */
+
+// Base class for the off-thread compile or off-thread decode tasks.
+class CompileOrDecodeTask : public mozilla::Task {
+ protected:
+  CompileOrDecodeTask();
+  virtual ~CompileOrDecodeTask();
+
+  nsresult InitFrontendContext();
+
+  void DidRunTask(const MutexAutoLock& aProofOfLock,
+                  RefPtr<JS::Stencil>&& aStencil);
+
+  bool IsCancelled(const MutexAutoLock& aProofOfLock) const {
+    return mIsCancelled;
+  }
+
+ public:
+  // Returns the result of the compilation or decode if it was successful.
+  // Returns nullptr otherwise, and sets pending exception on JSContext.
+  //
+  // aInstantiationStorage receives the storage allocated off main thread
+  // on successful case.
+  already_AddRefed<JS::Stencil> StealResult(
+      JSContext* aCx, JS::InstantiationStorage* aInstantiationStorage);
+
+  // Cancel the task.
+  // If the task is already running, this waits for the task to finish.
+  void Cancel();
+
+ protected:
+  // This mutex is locked during running the task or cancelling task.
+  mozilla::Mutex mMutex;
+
+  // The result of decode task, to distinguish throwing case and decode error.
+  JS::TranscodeResult mResult = JS::TranscodeResult::Ok;
+
+  // An option used to compile the code, or the equivalent for decode.
+  // This holds the filename pointed by errors reported to JS::FrontendContext.
+  JS::OwningCompileOptions mOptions;
+
+  // Owning-pointer for the context associated with the script compilation.
+  //
+  // The context is allocated on main thread in InitFrontendContext method,
+  // and is freed on any thread in the destructor.
+  JS::FrontendContext* mFrontendContext = nullptr;
+
+  bool mIsCancelled = false;
+
+ private:
+  // The result of the compilation or decode.
+  RefPtr<JS::Stencil> mStencil;
+
+  JS::InstantiationStorage mInstantiationStorage;
+};
+
+class ScriptLoadContext : public JS::loader::LoadContextBase,
+                          public PreloaderBase {
+ protected:
+  virtual ~ScriptLoadContext();
+
+ public:
+  explicit ScriptLoadContext();
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ScriptLoadContext,
+                                           JS::loader::LoadContextBase)
+
+  static void PrioritizeAsPreload(nsIChannel* aChannel);
+
+  bool IsPreload() const;
+
+  bool CompileStarted() const;
+
+  bool IsTracking() const { return mIsTracking; }
+  void SetIsTracking() {
+    MOZ_ASSERT(!mIsTracking);
+    mIsTracking = true;
+  }
+
+  void BlockOnload(Document* aDocument);
+
+  void MaybeUnblockOnload();
+
+  enum class ScriptMode : uint8_t {
+    eBlocking,
+    eDeferred,
+    eAsync,
+    eLinkPreload  // this is a load initiated by <link rel="preload"
+                  // as="script"> or <link rel="modulepreload"> tag
+  };
+
+  void SetScriptMode(bool aDeferAttr, bool aAsyncAttr, bool aLinkPreload);
+
+  bool IsLinkPreloadScript() const {
+    return mScriptMode == ScriptMode::eLinkPreload;
+  }
+
+  bool IsBlockingScript() const { return mScriptMode == ScriptMode::eBlocking; }
+
+  bool IsDeferredScript() const { return mScriptMode == ScriptMode::eDeferred; }
+
+  bool IsAsyncScript() const { return mScriptMode == ScriptMode::eAsync; }
+
+  nsIScriptElement* GetScriptElement() const;
+
+  // Make this request a preload (speculative) request.
+  void SetIsPreloadRequest() {
+    MOZ_ASSERT(!GetScriptElement());
+    MOZ_ASSERT(!IsPreload());
+    mIsPreload = true;
+  }
+
+  // Make a preload request into an actual load request for the given element.
+  void SetIsLoadRequest(nsIScriptElement* aElement);
+
+  FromParser GetParserCreated() const {
+    nsIScriptElement* element = GetScriptElement();
+    if (!element) {
+      return NOT_FROM_PARSER;
+    }
+    return element->GetParserCreated();
+  }
+
+  // Used to output a string for the Gecko Profiler.
+  void GetProfilerLabel(nsACString& aOutString) override;
+
+  void MaybeCancelOffThreadScript();
+
+  // Finish the off-main-thread compilation and return the result, or
+  // convert the compilation error to runtime error.
+  already_AddRefed<JS::Stencil> StealOffThreadResult(
+      JSContext* aCx, JS::InstantiationStorage* aInstantiationStorage);
+
+  ScriptMode mScriptMode;  // Whether this is a blocking, defer or async script.
+  bool mScriptFromHead;    // Synchronous head script block loading of other non
+                           // js/css content.
+  bool mIsInline;          // Is the script inline or loaded?
+  bool mInDeferList;       // True if we live in mDeferRequests.
+  bool mInAsyncList;       // True if we live in mLoadingAsyncRequests or
+                           // mLoadedAsyncRequests.
+  bool mIsNonAsyncScriptInserted;  // True if we live in
+                                   // mNonAsyncExternalScriptInsertedRequests
+  bool mIsXSLT;                    // True if we live in mXSLTRequests.
+  bool mInCompilingList;  // True if we are in mOffThreadCompilingRequests.
+  bool mIsTracking;       // True if the script comes from a source on our
+                          // tracking protection list.
+  bool mWasCompiledOMT;   // True if the script has been compiled off main
+                          // thread.
+
+  // Task that performs off-thread compilation or off-thread decode.
+  // This field is used to take the result of the task, or cancel the task.
+  //
+  // Set to non-null on the task creation, and set to null when taking the
+  // result or cancelling the task.
+  RefPtr<CompileOrDecodeTask> mCompileOrDecodeTask;
+
+  uint32_t mLineNo;
+  JS::ColumnNumberOneOrigin mColumnNo;
+
+  // Set on scripts and top level modules.
+  bool mIsPreload;
+
+  // Non-null if there is a document that this request is blocking from loading.
+  RefPtr<Document> mLoadBlockedDocument;
+
+  // For preload requests, we defer reporting errors to the console until the
+  // request is used.
+  nsresult mUnreportedPreloadError;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_ScriptLoadContext_h
diff --git a/dom/script/ScriptLoadHandler.cpp b/dom/script/ScriptLoadHandler.cpp
new file mode 100644
index 0000000000..08210c0c21
--- /dev/null
+++ b/dom/script/ScriptLoadHandler.cpp
@@ -0,0 +1,473 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ScriptLoadHandler.h"
+
+#include <stdlib.h>
+#include <utility>
+#include "ScriptCompression.h"
+#include "ScriptLoader.h"
+#include "ScriptTrace.h"
+#include "js/Transcoding.h"
+#include "js/loader/ScriptLoadRequest.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Encoding.h"
+#include "mozilla/Logging.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/PerfStats.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/Utf8.h"
+#include "mozilla/Vector.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/SRICheck.h"
+#include "mozilla/dom/ScriptDecoding.h"
+#include "nsCOMPtr.h"
+#include "nsContentUtils.h"
+#include "nsDebug.h"
+#include "nsICacheInfoChannel.h"
+#include "nsIChannel.h"
+#include "nsIHttpChannel.h"
+#include "nsIRequest.h"
+#include "nsIScriptElement.h"
+#include "nsIURI.h"
+#include "nsJSUtils.h"
+#include "nsMimeTypes.h"
+#include "nsString.h"
+#include "nsTArray.h"
+#include "zlib.h"
+
+namespace mozilla::dom {
+
+#undef LOG
+#define LOG(args) \
+  MOZ_LOG(ScriptLoader::gScriptLoaderLog, mozilla::LogLevel::Debug, args)
+
+#define LOG_ENABLED() \
+  MOZ_LOG_TEST(ScriptLoader::gScriptLoaderLog, mozilla::LogLevel::Debug)
+
+ScriptDecoder::ScriptDecoder(const Encoding* aEncoding,
+                             ScriptDecoder::BOMHandling handleBOM) {
+  if (handleBOM == BOMHandling::Ignore) {
+    mDecoder = aEncoding->NewDecoderWithoutBOMHandling();
+  } else {
+    mDecoder = aEncoding->NewDecoderWithBOMRemoval();
+  }
+  MOZ_ASSERT(mDecoder);
+}
+
+template <typename Unit>
+nsresult ScriptDecoder::DecodeRawDataHelper(
+    JS::loader::ScriptLoadRequest* aRequest, const uint8_t* aData,
+    uint32_t aDataLength, bool aEndOfStream) {
+  CheckedInt<size_t> needed =
+      ScriptDecoding<Unit>::MaxBufferLength(mDecoder, aDataLength);
+  if (!needed.isValid()) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  // Reference to the script source buffer which we will update.
+  JS::loader::ScriptLoadRequest::ScriptTextBuffer<Unit>& scriptText =
+      aRequest->ScriptText<Unit>();
+
+  uint32_t haveRead = scriptText.length();
+
+  CheckedInt<uint32_t> capacity = haveRead;
+  capacity += needed.value();
+
+  if (!capacity.isValid() || !scriptText.resize(capacity.value())) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  size_t written = ScriptDecoding<Unit>::DecodeInto(
+      mDecoder, Span(aData, aDataLength),
+      Span(scriptText.begin() + haveRead, needed.value()), aEndOfStream);
+  MOZ_ASSERT(written <= needed.value());
+
+  haveRead += written;
+  MOZ_ASSERT(haveRead <= capacity.value(),
+             "mDecoder produced more data than expected");
+  MOZ_ALWAYS_TRUE(scriptText.resize(haveRead));
+  aRequest->SetReceivedScriptTextLength(scriptText.length());
+
+  return NS_OK;
+}
+
+nsresult ScriptDecoder::DecodeRawData(JS::loader::ScriptLoadRequest* aRequest,
+                                      const uint8_t* aData,
+                                      uint32_t aDataLength, bool aEndOfStream) {
+  if (aRequest->IsUTF16Text()) {
+    return DecodeRawDataHelper<char16_t>(aRequest, aData, aDataLength,
+                                         aEndOfStream);
+  }
+
+  return DecodeRawDataHelper<Utf8Unit>(aRequest, aData, aDataLength,
+                                       aEndOfStream);
+}
+
+ScriptLoadHandler::ScriptLoadHandler(
+    ScriptLoader* aScriptLoader, JS::loader::ScriptLoadRequest* aRequest,
+    UniquePtr<SRICheckDataVerifier>&& aSRIDataVerifier)
+    : mScriptLoader(aScriptLoader),
+      mRequest(aRequest),
+      mSRIDataVerifier(std::move(aSRIDataVerifier)),
+      mSRIStatus(NS_OK) {
+  MOZ_ASSERT(aRequest->IsUnknownDataType());
+  MOZ_ASSERT(aRequest->IsFetching());
+}
+
+ScriptLoadHandler::~ScriptLoadHandler() = default;
+
+NS_IMPL_ISUPPORTS(ScriptLoadHandler, nsIIncrementalStreamLoaderObserver)
+
+NS_IMETHODIMP
+ScriptLoadHandler::OnIncrementalData(nsIIncrementalStreamLoader* aLoader,
+                                     nsISupports* aContext,
+                                     uint32_t aDataLength, const uint8_t* aData,
+                                     uint32_t* aConsumedLength) {
+  nsCOMPtr<nsIRequest> channelRequest;
+  aLoader->GetRequest(getter_AddRefs(channelRequest));
+
+  auto firstTime = !mPreloadStartNotified;
+  if (!mPreloadStartNotified) {
+    mPreloadStartNotified = true;
+    mRequest->GetScriptLoadContext()->NotifyStart(channelRequest);
+  }
+
+  if (mRequest->IsCanceled()) {
+    // If request cancelled, ignore any incoming data.
+    *aConsumedLength = aDataLength;
+    return NS_OK;
+  }
+
+  nsresult rv = NS_OK;
+  if (mRequest->IsUnknownDataType()) {
+    rv = EnsureKnownDataType(aLoader);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  if (mRequest->IsBytecode() && firstTime) {
+    PerfStats::RecordMeasurementStart(PerfStats::Metric::JSBC_IO_Read);
+  }
+
+  if (mRequest->IsTextSource()) {
+    if (!EnsureDecoder(aLoader, aData, aDataLength,
+                       /* aEndOfStream = */ false)) {
+      return NS_OK;
+    }
+
+    // Below we will/shall consume entire data chunk.
+    *aConsumedLength = aDataLength;
+
+    // Decoder has already been initialized. -- trying to decode all loaded
+    // bytes.
+    rv = mDecoder->DecodeRawData(mRequest, aData, aDataLength,
+                                 /* aEndOfStream = */ false);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    // If SRI is required for this load, appending new bytes to the hash.
+    if (mSRIDataVerifier && NS_SUCCEEDED(mSRIStatus)) {
+      mSRIStatus = mSRIDataVerifier->Update(aDataLength, aData);
+    }
+  } else {
+    MOZ_ASSERT(mRequest->IsBytecode());
+    if (!mRequest->SRIAndBytecode().append(aData, aDataLength)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    *aConsumedLength = aDataLength;
+    uint32_t sriLength = 0;
+    rv = MaybeDecodeSRI(&sriLength);
+    if (NS_FAILED(rv)) {
+      return channelRequest->Cancel(mScriptLoader->RestartLoad(mRequest));
+    }
+    if (sriLength) {
+      mRequest->SetSRILength(sriLength);
+    }
+  }
+
+  return rv;
+}
+
+bool ScriptLoadHandler::TrySetDecoder(nsIIncrementalStreamLoader* aLoader,
+                                      const uint8_t* aData,
+                                      uint32_t aDataLength, bool aEndOfStream) {
+  MOZ_ASSERT(mDecoder == nullptr,
+             "can't have a decoder already if we're trying to set one");
+
+  // JavaScript modules are always UTF-8.
+  if (mRequest->IsModuleRequest()) {
+    mDecoder = MakeUnique<ScriptDecoder>(UTF_8_ENCODING,
+                                         ScriptDecoder::BOMHandling::Remove);
+    return true;
+  }
+
+  // Determine if BOM check should be done.  This occurs either
+  // if end-of-stream has been reached, or at least 3 bytes have
+  // been read from input.
+  if (!aEndOfStream && (aDataLength < 3)) {
+    return false;
+  }
+
+  // Do BOM detection.
+  const Encoding* encoding;
+  std::tie(encoding, std::ignore) = Encoding::ForBOM(Span(aData, aDataLength));
+  if (encoding) {
+    mDecoder =
+        MakeUnique<ScriptDecoder>(encoding, ScriptDecoder::BOMHandling::Remove);
+    return true;
+  }
+
+  // BOM detection failed, check content stream for charset.
+  nsCOMPtr<nsIRequest> req;
+  nsresult rv = aLoader->GetRequest(getter_AddRefs(req));
+  NS_ASSERTION(req, "StreamLoader's request went away prematurely");
+  NS_ENSURE_SUCCESS(rv, false);
+
+  nsCOMPtr<nsIChannel> channel = do_QueryInterface(req);
+
+  if (channel) {
+    nsAutoCString label;
+    if (NS_SUCCEEDED(channel->GetContentCharset(label)) &&
+        (encoding = Encoding::ForLabel(label))) {
+      mDecoder = MakeUnique<ScriptDecoder>(encoding,
+                                           ScriptDecoder::BOMHandling::Ignore);
+      return true;
+    }
+  }
+
+  // Check the hint charset from the script element or preload
+  // request.
+  nsAutoString hintCharset;
+  if (!mRequest->GetScriptLoadContext()->IsPreload()) {
+    mRequest->GetScriptLoadContext()->GetScriptElement()->GetScriptCharset(
+        hintCharset);
+  } else {
+    nsTArray<ScriptLoader::PreloadInfo>::index_type i =
+        mScriptLoader->mPreloads.IndexOf(
+            mRequest, 0, ScriptLoader::PreloadRequestComparator());
+
+    NS_ASSERTION(i != mScriptLoader->mPreloads.NoIndex,
+                 "Incorrect preload bookkeeping");
+    hintCharset = mScriptLoader->mPreloads[i].mCharset;
+  }
+
+  if ((encoding = Encoding::ForLabel(hintCharset))) {
+    mDecoder =
+        MakeUnique<ScriptDecoder>(encoding, ScriptDecoder::BOMHandling::Ignore);
+    return true;
+  }
+
+  // Get the charset from the charset of the document.
+  if (mScriptLoader->mDocument) {
+    encoding = mScriptLoader->mDocument->GetDocumentCharacterSet();
+    mDecoder =
+        MakeUnique<ScriptDecoder>(encoding, ScriptDecoder::BOMHandling::Ignore);
+    return true;
+  }
+
+  // Curiously, there are various callers that don't pass aDocument. The
+  // fallback in the old code was ISO-8859-1, which behaved like
+  // windows-1252.
+  mDecoder = MakeUnique<ScriptDecoder>(WINDOWS_1252_ENCODING,
+                                       ScriptDecoder::BOMHandling::Ignore);
+  return true;
+}
+
+nsresult ScriptLoadHandler::MaybeDecodeSRI(uint32_t* sriLength) {
+  *sriLength = 0;
+
+  if (!mSRIDataVerifier || mSRIDataVerifier->IsComplete() ||
+      NS_FAILED(mSRIStatus)) {
+    return NS_OK;
+  }
+
+  // Skip until the content is large enough to be decoded.
+  JS::TranscodeBuffer& receivedData = mRequest->SRIAndBytecode();
+  if (receivedData.length() <= mSRIDataVerifier->DataSummaryLength()) {
+    return NS_OK;
+  }
+
+  mSRIStatus = mSRIDataVerifier->ImportDataSummary(receivedData.length(),
+                                                   receivedData.begin());
+
+  if (NS_FAILED(mSRIStatus)) {
+    // We are unable to decode the hash contained in the alternate data which
+    // contains the bytecode, or it does not use the same algorithm.
+    LOG(
+        ("ScriptLoadHandler::MaybeDecodeSRI, failed to decode SRI, restart "
+         "request"));
+    return mSRIStatus;
+  }
+
+  *sriLength = mSRIDataVerifier->DataSummaryLength();
+  MOZ_ASSERT(*sriLength > 0);
+  return NS_OK;
+}
+
+nsresult ScriptLoadHandler::EnsureKnownDataType(
+    nsIIncrementalStreamLoader* aLoader) {
+  MOZ_ASSERT(mRequest->IsUnknownDataType());
+  MOZ_ASSERT(mRequest->IsFetching());
+
+  nsCOMPtr<nsIRequest> req;
+  nsresult rv = aLoader->GetRequest(getter_AddRefs(req));
+  MOZ_ASSERT(req, "StreamLoader's request went away prematurely");
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (mRequest->mFetchSourceOnly) {
+    mRequest->SetTextSource(mRequest->mLoadContext.get());
+    TRACE_FOR_TEST(mRequest->GetScriptLoadContext()->GetScriptElement(),
+                   "scriptloader_load_source");
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsICacheInfoChannel> cic(do_QueryInterface(req));
+  if (cic) {
+    nsAutoCString altDataType;
+    cic->GetAlternativeDataType(altDataType);
+    if (altDataType.Equals(ScriptLoader::BytecodeMimeTypeFor(mRequest))) {
+      mRequest->SetBytecode();
+      TRACE_FOR_TEST(mRequest->GetScriptLoadContext()->GetScriptElement(),
+                     "scriptloader_load_bytecode");
+      return NS_OK;
+    }
+    MOZ_ASSERT(altDataType.IsEmpty());
+  }
+
+  mRequest->SetTextSource(mRequest->mLoadContext.get());
+  TRACE_FOR_TEST(mRequest->GetScriptLoadContext()->GetScriptElement(),
+                 "scriptloader_load_source");
+
+  MOZ_ASSERT(!mRequest->IsUnknownDataType());
+  MOZ_ASSERT(mRequest->IsFetching());
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+ScriptLoadHandler::OnStreamComplete(nsIIncrementalStreamLoader* aLoader,
+                                    nsISupports* aContext, nsresult aStatus,
+                                    uint32_t aDataLength,
+                                    const uint8_t* aData) {
+  nsresult rv = NS_OK;
+  if (LOG_ENABLED()) {
+    nsAutoCString url;
+    mRequest->mURI->GetAsciiSpec(url);
+    LOG(("ScriptLoadRequest (%p): Stream complete (url = %s)", mRequest.get(),
+         url.get()));
+  }
+
+  nsCOMPtr<nsIRequest> channelRequest;
+  aLoader->GetRequest(getter_AddRefs(channelRequest));
+
+  auto firstMessage = !mPreloadStartNotified;
+  if (!mPreloadStartNotified) {
+    mPreloadStartNotified = true;
+    mRequest->GetScriptLoadContext()->NotifyStart(channelRequest);
+  }
+
+  auto notifyStop = MakeScopeExit([&] {
+    mRequest->GetScriptLoadContext()->NotifyStop(channelRequest, rv);
+  });
+
+  if (!mRequest->IsCanceled()) {
+    if (mRequest->IsUnknownDataType()) {
+      rv = EnsureKnownDataType(aLoader);
+      NS_ENSURE_SUCCESS(rv, rv);
+    }
+
+    if (mRequest->IsBytecode() && !firstMessage) {
+      // if firstMessage, then entire stream is in aData, and PerfStats would
+      // measure 0 time
+      PerfStats::RecordMeasurementEnd(PerfStats::Metric::JSBC_IO_Read);
+    }
+
+    if (mRequest->IsTextSource()) {
+      DebugOnly<bool> encoderSet =
+          EnsureDecoder(aLoader, aData, aDataLength, /* aEndOfStream = */ true);
+      MOZ_ASSERT(encoderSet);
+      rv = mDecoder->DecodeRawData(mRequest, aData, aDataLength,
+                                   /* aEndOfStream = */ true);
+      NS_ENSURE_SUCCESS(rv, rv);
+
+      LOG(("ScriptLoadRequest (%p): Source length in code units = %u",
+           mRequest.get(), unsigned(mRequest->ScriptTextLength())));
+
+      // If SRI is required for this load, appending new bytes to the hash.
+      if (mSRIDataVerifier && NS_SUCCEEDED(mSRIStatus)) {
+        mSRIStatus = mSRIDataVerifier->Update(aDataLength, aData);
+      }
+    } else {
+      MOZ_ASSERT(mRequest->IsBytecode());
+      JS::TranscodeBuffer& bytecode = mRequest->SRIAndBytecode();
+      if (!bytecode.append(aData, aDataLength)) {
+        return NS_ERROR_OUT_OF_MEMORY;
+      }
+
+      LOG(("ScriptLoadRequest (%p): Bytecode length = %u", mRequest.get(),
+           unsigned(bytecode.length())));
+
+      // If we abort while decoding the SRI, we fallback on explictly requesting
+      // the source. Thus, we should not continue in
+      // ScriptLoader::OnStreamComplete, which removes the request from the
+      // waiting lists.
+      //
+      // We calculate the SRI length below.
+      uint32_t unused;
+      rv = MaybeDecodeSRI(&unused);
+      if (NS_FAILED(rv)) {
+        return channelRequest->Cancel(mScriptLoader->RestartLoad(mRequest));
+      }
+
+      // The bytecode cache always starts with the SRI hash, thus even if there
+      // is no SRI data verifier instance, we still want to skip the hash.
+      uint32_t sriLength;
+      rv = SRICheckDataVerifier::DataSummaryLength(
+          bytecode.length(), bytecode.begin(), &sriLength);
+      if (NS_FAILED(rv)) {
+        return channelRequest->Cancel(mScriptLoader->RestartLoad(mRequest));
+      }
+
+      mRequest->SetSRILength(sriLength);
+
+      Vector<uint8_t> compressedBytecode;
+      // mRequest has the compressed bytecode, but will be filled with the
+      // uncompressed bytecode
+      compressedBytecode.swap(bytecode);
+      if (!JS::loader::ScriptBytecodeDecompress(
+              compressedBytecode, mRequest->GetSRILength(), bytecode)) {
+        return NS_ERROR_UNEXPECTED;
+      }
+    }
+  }
+
+  // Everything went well, keep the CacheInfoChannel alive such that we can
+  // later save the bytecode on the cache entry.
+  if (NS_SUCCEEDED(rv) && mRequest->IsSource() &&
+      StaticPrefs::dom_script_loader_bytecode_cache_enabled()) {
+    mRequest->mCacheInfo = do_QueryInterface(channelRequest);
+    LOG(("ScriptLoadRequest (%p): nsICacheInfoChannel = %p", mRequest.get(),
+         mRequest->mCacheInfo.get()));
+  }
+
+  // we have to mediate and use mRequest.
+  rv = mScriptLoader->OnStreamComplete(aLoader, mRequest, aStatus, mSRIStatus,
+                                       mSRIDataVerifier.get());
+
+  // In case of failure, clear the mCacheInfoChannel to avoid keeping it alive.
+  if (NS_FAILED(rv)) {
+    mRequest->mCacheInfo = nullptr;
+  }
+
+  return rv;
+}
+
+#undef LOG_ENABLED
+#undef LOG
+
+}  // namespace mozilla::dom
diff --git a/dom/script/ScriptLoadHandler.h b/dom/script/ScriptLoadHandler.h
new file mode 100644
index 0000000000..8357ad75a8
--- /dev/null
+++ b/dom/script/ScriptLoadHandler.h
@@ -0,0 +1,136 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * A class that handles loading and evaluation of <script> elements.
+ */
+
+#ifndef mozilla_dom_ScriptLoadHandler_h
+#define mozilla_dom_ScriptLoadHandler_h
+
+#include "nsIIncrementalStreamLoader.h"
+#include "nsISupports.h"
+#include "mozilla/Encoding.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
+
+namespace JS::loader {
+class ScriptLoadRequest;
+}
+
+namespace mozilla {
+
+class Decoder;
+
+namespace dom {
+
+class ScriptLoader;
+class SRICheckDataVerifier;
+
+class ScriptDecoder {
+ public:
+  enum BOMHandling { Ignore, Remove };
+
+  ScriptDecoder(const Encoding* aEncoding,
+                ScriptDecoder::BOMHandling handleBOM);
+
+  ~ScriptDecoder() = default;
+
+  /*
+   * Once the charset is found by the EnsureDecoder function, we can
+   * incrementally convert the charset to the one expected by the JS Parser.
+   */
+  nsresult DecodeRawData(JS::loader::ScriptLoadRequest* aRequest,
+                         const uint8_t* aData, uint32_t aDataLength,
+                         bool aEndOfStream);
+
+ private:
+  /*
+   * Decode the given data into the already-allocated internal
+   * |ScriptTextBuffer<Unit>|.
+   *
+   * This function is intended to be called only by |DecodeRawData| after
+   * determining which sort of |ScriptTextBuffer<Unit>| has been allocated.
+   */
+  template <typename Unit>
+  nsresult DecodeRawDataHelper(JS::loader::ScriptLoadRequest* aRequest,
+                               const uint8_t* aData, uint32_t aDataLength,
+                               bool aEndOfStream);
+
+  // Unicode decoder for charset.
+  mozilla::UniquePtr<mozilla::Decoder> mDecoder;
+};
+
+class ScriptLoadHandler final : public nsIIncrementalStreamLoaderObserver {
+ public:
+  explicit ScriptLoadHandler(
+      ScriptLoader* aScriptLoader, JS::loader::ScriptLoadRequest* aRequest,
+      UniquePtr<SRICheckDataVerifier>&& aSRIDataVerifier);
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIINCREMENTALSTREAMLOADEROBSERVER
+
+ private:
+  virtual ~ScriptLoadHandler();
+
+  /*
+   * Discover the charset by looking at the stream data, the script tag, and
+   * other indicators.  Returns true if charset has been discovered.
+   */
+  bool EnsureDecoder(nsIIncrementalStreamLoader* aLoader, const uint8_t* aData,
+                     uint32_t aDataLength, bool aEndOfStream) {
+    // Check if the decoder has already been created.
+    if (mDecoder) {
+      return true;
+    }
+
+    return TrySetDecoder(aLoader, aData, aDataLength, aEndOfStream);
+  }
+
+  /*
+   * Attempt to determine how script data will be decoded, when such
+   * determination hasn't already been made.  (If you don't know whether it's
+   * been made yet, use |EnsureDecoder| above instead.)  Return false if there
+   * isn't enough information yet to make the determination, or true if a
+   * determination was made.
+   */
+  bool TrySetDecoder(nsIIncrementalStreamLoader* aLoader, const uint8_t* aData,
+                     uint32_t aDataLength, bool aEndOfStream);
+
+  /*
+   * When streaming bytecode, we have the opportunity to fallback early if SRI
+   * does not match the expectation of the document.
+   *
+   * If SRI hash is decoded, `sriLength` is set to the length of the hash.
+   */
+  nsresult MaybeDecodeSRI(uint32_t* sriLength);
+
+  // Query the channel to find the data type associated with the input stream.
+  nsresult EnsureKnownDataType(nsIIncrementalStreamLoader* aLoader);
+
+  // ScriptLoader which will handle the parsed script.
+  RefPtr<ScriptLoader> mScriptLoader;
+
+  // The ScriptLoadRequest for this load. Decoded data are accumulated on it.
+  RefPtr<JS::loader::ScriptLoadRequest> mRequest;
+
+  // SRI data verifier.
+  UniquePtr<SRICheckDataVerifier> mSRIDataVerifier;
+
+  // Status of SRI data operations.
+  nsresult mSRIStatus;
+
+  UniquePtr<ScriptDecoder> mDecoder;
+
+  // Flipped to true after calling NotifyStart the first time
+  bool mPreloadStartNotified = false;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_ScriptLoadHandler_h
diff --git a/dom/script/ScriptLoader.cpp b/dom/script/ScriptLoader.cpp
new file mode 100644
index 0000000000..43f718ab64
--- /dev/null
+++ b/dom/script/ScriptLoader.cpp
@@ -0,0 +1,4110 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ScriptLoader.h"
+#include "ScriptLoadHandler.h"
+#include "ScriptTrace.h"
+#include "ModuleLoader.h"
+#include "nsGenericHTMLElement.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/dom/FetchPriority.h"
+#include "mozilla/dom/RequestBinding.h"
+#include "nsIChildChannel.h"
+#include "zlib.h"
+
+#include "prsystem.h"
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "js/Array.h"         // JS::GetArrayLength
+#include "js/ColumnNumber.h"  // JS::ColumnNumberOneOrigin
+#include "js/CompilationAndEvaluation.h"
+#include "js/CompileOptions.h"  // JS::CompileOptions, JS::OwningCompileOptions, JS::DecodeOptions, JS::OwningDecodeOptions, JS::DelazificationOption
+#include "js/ContextOptions.h"  // JS::ContextOptionsRef
+#include "js/experimental/JSStencil.h"  // JS::Stencil, JS::InstantiationStorage
+#include "js/experimental/CompileScript.h"  // JS::FrontendContext, JS::NewFrontendContext, JS::DestroyFrontendContext, JS::SetNativeStackQuota, JS::ThreadStackQuotaForSize, JS::CompilationStorage, JS::CompileGlobalScriptToStencil, JS::CompileModuleScriptToStencil, JS::DecodeStencil, JS::PrepareForInstantiate
+#include "js/friend/ErrorMessages.h"        // js::GetErrorMessage, JSMSG_*
+#include "js/loader/ScriptLoadRequest.h"
+#include "ScriptCompression.h"
+#include "js/loader/LoadedScript.h"
+#include "js/loader/ModuleLoadRequest.h"
+#include "js/MemoryFunctions.h"
+#include "js/Modules.h"
+#include "js/PropertyAndElement.h"  // JS_DefineProperty
+#include "js/Realm.h"
+#include "js/SourceText.h"
+#include "js/Transcoding.h"  // JS::TranscodeRange, JS::TranscodeResult, JS::IsTranscodeFailureResult
+#include "js/Utility.h"
+#include "xpcpublic.h"
+#include "GeckoProfiler.h"
+#include "nsContentSecurityManager.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIContent.h"
+#include "nsJSUtils.h"
+#include "mozilla/dom/AutoEntryScript.h"
+#include "mozilla/dom/DocGroup.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/JSExecutionContext.h"
+#include "mozilla/dom/ScriptDecoding.h"  // mozilla::dom::ScriptDecoding
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/dom/SRILogHelper.h"
+#include "mozilla/dom/WindowContext.h"
+#include "mozilla/Mutex.h"  // mozilla::Mutex
+#include "mozilla/net/UrlClassifierFeatureFactory.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/StaticPrefs_javascript.h"
+#include "mozilla/StaticPrefs_network.h"
+#include "nsAboutProtocolUtils.h"
+#include "nsGkAtoms.h"
+#include "nsNetUtil.h"
+#include "nsIScriptGlobalObject.h"
+#include "nsIScriptContext.h"
+#include "nsIPrincipal.h"
+#include "nsJSPrincipals.h"
+#include "nsContentPolicyUtils.h"
+#include "nsContentSecurityUtils.h"
+#include "nsIClassifiedChannel.h"
+#include "nsIHttpChannel.h"
+#include "nsIHttpChannelInternal.h"
+#include "nsIClassOfService.h"
+#include "nsICacheInfoChannel.h"
+#include "nsITimedChannel.h"
+#include "nsIScriptElement.h"
+#include "nsISupportsPriority.h"
+#include "nsIDocShell.h"
+#include "nsContentUtils.h"
+#include "nsUnicharUtils.h"
+#include "nsError.h"
+#include "nsThreadUtils.h"
+#include "nsDocShellCID.h"
+#include "nsIContentSecurityPolicy.h"
+#include "mozilla/Logging.h"
+#include "nsCRT.h"
+#include "nsContentCreatorFunctions.h"
+#include "nsProxyRelease.h"
+#include "nsSandboxFlags.h"
+#include "nsContentTypeParser.h"
+#include "nsINetworkPredictor.h"
+#include "nsMimeTypes.h"
+#include "mozilla/ConsoleReportCollector.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/EventQueue.h"
+#include "mozilla/LoadInfo.h"
+#include "ReferrerInfo.h"
+
+#include "mozilla/AsyncEventDispatcher.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/TaskController.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Unused.h"
+#include "mozilla/Utf8.h"  // mozilla::Utf8Unit
+#include "nsIScriptError.h"
+#include "nsIAsyncOutputStream.h"
+#include "js/loader/ModuleLoaderBase.h"
+#include "mozilla/Maybe.h"
+
+using JS::SourceText;
+using namespace JS::loader;
+
+using mozilla::Telemetry::LABELS_DOM_SCRIPT_PRELOAD_RESULT;
+
+namespace mozilla::dom {
+
+LazyLogModule ScriptLoader::gCspPRLog("CSP");
+LazyLogModule ScriptLoader::gScriptLoaderLog("ScriptLoader");
+
+#undef LOG
+#define LOG(args) \
+  MOZ_LOG(ScriptLoader::gScriptLoaderLog, mozilla::LogLevel::Debug, args)
+
+#define LOG_ENABLED() \
+  MOZ_LOG_TEST(ScriptLoader::gScriptLoaderLog, mozilla::LogLevel::Debug)
+
+// Alternate Data MIME type used by the ScriptLoader to register that we want to
+// store bytecode without reading it.
+static constexpr auto kNullMimeType = "javascript/null"_ns;
+
+/////////////////////////////////////////////////////////////
+// AsyncCompileShutdownObserver
+/////////////////////////////////////////////////////////////
+
+NS_IMPL_ISUPPORTS(AsyncCompileShutdownObserver, nsIObserver)
+
+void AsyncCompileShutdownObserver::OnShutdown() {
+  if (mScriptLoader) {
+    mScriptLoader->Destroy();
+    MOZ_ASSERT(!mScriptLoader);
+  }
+}
+
+void AsyncCompileShutdownObserver::Unregister() {
+  if (mScriptLoader) {
+    mScriptLoader = nullptr;
+    nsContentUtils::UnregisterShutdownObserver(this);
+  }
+}
+
+NS_IMETHODIMP
+AsyncCompileShutdownObserver::Observe(nsISupports* aSubject, const char* aTopic,
+                                      const char16_t* aData) {
+  OnShutdown();
+  return NS_OK;
+}
+
+//////////////////////////////////////////////////////////////
+// ScriptLoader::PreloadInfo
+//////////////////////////////////////////////////////////////
+
+inline void ImplCycleCollectionUnlink(ScriptLoader::PreloadInfo& aField) {
+  ImplCycleCollectionUnlink(aField.mRequest);
+}
+
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    ScriptLoader::PreloadInfo& aField, const char* aName, uint32_t aFlags = 0) {
+  ImplCycleCollectionTraverse(aCallback, aField.mRequest, aName, aFlags);
+}
+
+//////////////////////////////////////////////////////////////
+// ScriptLoader
+//////////////////////////////////////////////////////////////
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ScriptLoader)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION(ScriptLoader, mNonAsyncExternalScriptInsertedRequests,
+                         mLoadingAsyncRequests, mLoadedAsyncRequests,
+                         mOffThreadCompilingRequests, mDeferRequests,
+                         mXSLTRequests, mParserBlockingRequest,
+                         mBytecodeEncodingQueue, mPreloads,
+                         mPendingChildLoaders, mModuleLoader,
+                         mWebExtModuleLoaders, mShadowRealmModuleLoaders)
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ScriptLoader)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ScriptLoader)
+
+ScriptLoader::ScriptLoader(Document* aDocument)
+    : mDocument(aDocument),
+      mParserBlockingBlockerCount(0),
+      mBlockerCount(0),
+      mNumberOfProcessors(0),
+      mTotalFullParseSize(0),
+      mPhysicalSizeOfMemory(-1),
+      mEnabled(true),
+      mDeferEnabled(false),
+      mSpeculativeOMTParsingEnabled(false),
+      mDeferCheckpointReached(false),
+      mBlockingDOMContentLoaded(false),
+      mLoadEventFired(false),
+      mGiveUpEncoding(false),
+      mReporter(new ConsoleReportCollector()) {
+  LOG(("ScriptLoader::ScriptLoader %p", this));
+
+  mSpeculativeOMTParsingEnabled = StaticPrefs::
+      dom_script_loader_external_scripts_speculative_omt_parse_enabled();
+
+  mShutdownObserver = new AsyncCompileShutdownObserver(this);
+  nsContentUtils::RegisterShutdownObserver(mShutdownObserver);
+}
+
+ScriptLoader::~ScriptLoader() {
+  LOG(("ScriptLoader::~ScriptLoader %p", this));
+
+  mObservers.Clear();
+
+  if (mParserBlockingRequest) {
+    FireScriptAvailable(NS_ERROR_ABORT, mParserBlockingRequest);
+  }
+
+  for (ScriptLoadRequest* req = mXSLTRequests.getFirst(); req;
+       req = req->getNext()) {
+    FireScriptAvailable(NS_ERROR_ABORT, req);
+  }
+
+  for (ScriptLoadRequest* req = mDeferRequests.getFirst(); req;
+       req = req->getNext()) {
+    FireScriptAvailable(NS_ERROR_ABORT, req);
+  }
+
+  for (ScriptLoadRequest* req = mLoadingAsyncRequests.getFirst(); req;
+       req = req->getNext()) {
+    FireScriptAvailable(NS_ERROR_ABORT, req);
+  }
+
+  for (ScriptLoadRequest* req = mLoadedAsyncRequests.getFirst(); req;
+       req = req->getNext()) {
+    FireScriptAvailable(NS_ERROR_ABORT, req);
+  }
+
+  for (ScriptLoadRequest* req =
+           mNonAsyncExternalScriptInsertedRequests.getFirst();
+       req; req = req->getNext()) {
+    FireScriptAvailable(NS_ERROR_ABORT, req);
+  }
+
+  // Unblock the kids, in case any of them moved to a different document
+  // subtree in the meantime and therefore aren't actually going away.
+  for (uint32_t j = 0; j < mPendingChildLoaders.Length(); ++j) {
+    mPendingChildLoaders[j]->RemoveParserBlockingScriptExecutionBlocker();
+  }
+
+  for (size_t i = 0; i < mPreloads.Length(); i++) {
+    AccumulateCategorical(LABELS_DOM_SCRIPT_PRELOAD_RESULT::NotUsed);
+  }
+
+  if (mShutdownObserver) {
+    mShutdownObserver->Unregister();
+    mShutdownObserver = nullptr;
+  }
+
+  mModuleLoader = nullptr;
+}
+
+void ScriptLoader::SetGlobalObject(nsIGlobalObject* aGlobalObject) {
+  if (!aGlobalObject) {
+    // The document is being detached.
+    CancelAndClearScriptLoadRequests();
+    return;
+  }
+
+  MOZ_ASSERT(!HasPendingRequests());
+
+  if (!mModuleLoader) {
+    // The module loader is associated with a global object, so don't create it
+    // until we have a global set.
+    mModuleLoader = new ModuleLoader(this, aGlobalObject, ModuleLoader::Normal);
+  }
+
+  MOZ_ASSERT(mModuleLoader->GetGlobalObject() == aGlobalObject);
+  MOZ_ASSERT(aGlobalObject->GetModuleLoader(dom::danger::GetJSContext()) ==
+             mModuleLoader);
+}
+
+void ScriptLoader::RegisterContentScriptModuleLoader(ModuleLoader* aLoader) {
+  MOZ_ASSERT(aLoader);
+  MOZ_ASSERT(aLoader->GetScriptLoader() == this);
+
+  mWebExtModuleLoaders.AppendElement(aLoader);
+}
+
+void ScriptLoader::RegisterShadowRealmModuleLoader(ModuleLoader* aLoader) {
+  MOZ_ASSERT(aLoader);
+  MOZ_ASSERT(aLoader->GetScriptLoader() == this);
+
+  mShadowRealmModuleLoaders.AppendElement(aLoader);
+}
+
+// Collect telemtry data about the cache information, and the kind of source
+// which are being loaded, and where it is being loaded from.
+static void CollectScriptTelemetry(ScriptLoadRequest* aRequest) {
+  using namespace mozilla::Telemetry;
+
+  MOZ_ASSERT(aRequest->IsFetching());
+
+  // Skip this function if we are not running telemetry.
+  if (!CanRecordExtended()) {
+    return;
+  }
+
+  // Report the script kind.
+  if (aRequest->IsModuleRequest()) {
+    AccumulateCategorical(LABELS_DOM_SCRIPT_KIND::ModuleScript);
+  } else {
+    AccumulateCategorical(LABELS_DOM_SCRIPT_KIND::ClassicScript);
+  }
+
+  // Report the type of source. This is used to monitor the status of the
+  // JavaScript Start-up Bytecode Cache, with the expectation of an almost zero
+  // source-fallback and alternate-data being roughtly equal to source loads.
+  if (aRequest->mFetchSourceOnly) {
+    if (aRequest->GetScriptLoadContext()->mIsInline) {
+      AccumulateCategorical(LABELS_DOM_SCRIPT_LOADING_SOURCE::Inline);
+    } else if (aRequest->IsTextSource()) {
+      AccumulateCategorical(LABELS_DOM_SCRIPT_LOADING_SOURCE::SourceFallback);
+    }
+  } else {
+    if (aRequest->IsTextSource()) {
+      AccumulateCategorical(LABELS_DOM_SCRIPT_LOADING_SOURCE::Source);
+    } else if (aRequest->IsBytecode()) {
+      AccumulateCategorical(LABELS_DOM_SCRIPT_LOADING_SOURCE::AltData);
+    }
+  }
+}
+
+// Helper method for checking if the script element is an event-handler
+// This means that it has both a for-attribute and a event-attribute.
+// Also, if the for-attribute has a value that matches "\s*window\s*",
+// and the event-attribute matches "\s*onload([ \(].*)?" then it isn't an
+// eventhandler. (both matches are case insensitive).
+// This is how IE seems to filter out a window's onload handler from a
+// <script for=... event=...> element.
+
+static bool IsScriptEventHandler(ScriptKind kind, nsIContent* aScriptElement) {
+  if (kind != ScriptKind::eClassic) {
+    return false;
+  }
+
+  if (!aScriptElement->IsHTMLElement()) {
+    return false;
+  }
+
+  nsAutoString forAttr, eventAttr;
+  if (!aScriptElement->AsElement()->GetAttr(nsGkAtoms::_for, forAttr) ||
+      !aScriptElement->AsElement()->GetAttr(nsGkAtoms::event, eventAttr)) {
+    return false;
+  }
+
+  const nsAString& for_str =
+      nsContentUtils::TrimWhitespace<nsCRT::IsAsciiSpace>(forAttr);
+  if (!for_str.LowerCaseEqualsLiteral("window")) {
+    return true;
+  }
+
+  // We found for="window", now check for event="onload".
+  const nsAString& event_str =
+      nsContentUtils::TrimWhitespace<nsCRT::IsAsciiSpace>(eventAttr, false);
+  if (!StringBeginsWith(event_str, u"onload"_ns,
+                        nsCaseInsensitiveStringComparator)) {
+    // It ain't "onload.*".
+
+    return true;
+  }
+
+  nsAutoString::const_iterator start, end;
+  event_str.BeginReading(start);
+  event_str.EndReading(end);
+
+  start.advance(6);  // advance past "onload"
+
+  if (start != end && *start != '(' && *start != ' ') {
+    // We got onload followed by something other than space or
+    // '('. Not good enough.
+
+    return true;
+  }
+
+  return false;
+}
+
+nsContentPolicyType ScriptLoadRequestToContentPolicyType(
+    ScriptLoadRequest* aRequest) {
+  if (aRequest->GetScriptLoadContext()->IsPreload()) {
+    return aRequest->IsModuleRequest()
+               ? nsIContentPolicy::TYPE_INTERNAL_MODULE_PRELOAD
+               : nsIContentPolicy::TYPE_INTERNAL_SCRIPT_PRELOAD;
+  }
+
+  return aRequest->IsModuleRequest() ? nsIContentPolicy::TYPE_INTERNAL_MODULE
+                                     : nsIContentPolicy::TYPE_INTERNAL_SCRIPT;
+}
+
+nsresult ScriptLoader::CheckContentPolicy(Document* aDocument,
+                                          nsIScriptElement* aElement,
+                                          const nsAString& aNonce,
+                                          ScriptLoadRequest* aRequest) {
+  nsContentPolicyType contentPolicyType =
+      ScriptLoadRequestToContentPolicyType(aRequest);
+
+  nsCOMPtr<nsINode> requestingNode = do_QueryInterface(aElement);
+  nsCOMPtr<nsILoadInfo> secCheckLoadInfo = new net::LoadInfo(
+      aDocument->NodePrincipal(),  // loading principal
+      aDocument->NodePrincipal(),  // triggering principal
+      requestingNode, nsILoadInfo::SEC_ONLY_FOR_EXPLICIT_CONTENTSEC_CHECK,
+      contentPolicyType);
+  secCheckLoadInfo->SetParserCreatedScript(aElement->GetParserCreated() !=
+                                           mozilla::dom::NOT_FROM_PARSER);
+  // Use nonce of the current element, instead of the preload, because those
+  // are allowed to differ.
+  secCheckLoadInfo->SetCspNonce(aNonce);
+  if (aRequest->mIntegrity.IsValid()) {
+    MOZ_ASSERT(!aRequest->mIntegrity.IsEmpty());
+    secCheckLoadInfo->SetIntegrityMetadata(
+        aRequest->mIntegrity.GetIntegrityString());
+  }
+
+  int16_t shouldLoad = nsIContentPolicy::ACCEPT;
+  nsresult rv =
+      NS_CheckContentLoadPolicy(aRequest->mURI, secCheckLoadInfo, &shouldLoad,
+                                nsContentUtils::GetContentPolicy());
+  if (NS_FAILED(rv) || NS_CP_REJECTED(shouldLoad)) {
+    if (NS_FAILED(rv) || shouldLoad != nsIContentPolicy::REJECT_TYPE) {
+      return NS_ERROR_CONTENT_BLOCKED;
+    }
+    return NS_ERROR_CONTENT_BLOCKED_SHOW_ALT;
+  }
+
+  return NS_OK;
+}
+
+/* static */
+bool ScriptLoader::IsAboutPageLoadingChromeURI(ScriptLoadRequest* aRequest,
+                                               Document* aDocument) {
+  // if the uri to be loaded is not of scheme chrome:, there is nothing to do.
+  if (!aRequest->mURI->SchemeIs("chrome")) {
+    return false;
+  }
+
+  // we can either get here with a regular contentPrincipal or with a
+  // NullPrincipal in case we are showing an error page in a sandboxed iframe.
+  // In either case if the about: page is linkable from content, there is
+  // nothing to do.
+  uint32_t aboutModuleFlags = 0;
+  nsresult rv = NS_OK;
+
+  nsCOMPtr<nsIPrincipal> triggeringPrincipal = aRequest->TriggeringPrincipal();
+  if (triggeringPrincipal->GetIsContentPrincipal()) {
+    if (!triggeringPrincipal->SchemeIs("about")) {
+      return false;
+    }
+    rv = triggeringPrincipal->GetAboutModuleFlags(&aboutModuleFlags);
+    NS_ENSURE_SUCCESS(rv, false);
+  } else if (triggeringPrincipal->GetIsNullPrincipal()) {
+    nsCOMPtr<nsIURI> docURI = aDocument->GetDocumentURI();
+    if (!docURI->SchemeIs("about")) {
+      return false;
+    }
+
+    nsCOMPtr<nsIAboutModule> aboutModule;
+    rv = NS_GetAboutModule(docURI, getter_AddRefs(aboutModule));
+    if (NS_FAILED(rv) || !aboutModule) {
+      return false;
+    }
+    rv = aboutModule->GetURIFlags(docURI, &aboutModuleFlags);
+    NS_ENSURE_SUCCESS(rv, false);
+  } else {
+    return false;
+  }
+
+  if (aboutModuleFlags & nsIAboutModule::MAKE_LINKABLE) {
+    return false;
+  }
+
+  // seems like an about page wants to load a chrome URI.
+  return true;
+}
+
+nsIURI* ScriptLoader::GetBaseURI() const {
+  MOZ_ASSERT(mDocument);
+  return mDocument->GetDocBaseURI();
+}
+
+class ScriptRequestProcessor : public Runnable {
+ private:
+  RefPtr<ScriptLoader> mLoader;
+  RefPtr<ScriptLoadRequest> mRequest;
+
+ public:
+  ScriptRequestProcessor(ScriptLoader* aLoader, ScriptLoadRequest* aRequest)
+      : Runnable("dom::ScriptRequestProcessor"),
+        mLoader(aLoader),
+        mRequest(aRequest) {}
+  NS_IMETHOD Run() override { return mLoader->ProcessRequest(mRequest); }
+};
+
+void ScriptLoader::RunScriptWhenSafe(ScriptLoadRequest* aRequest) {
+  auto* runnable = new ScriptRequestProcessor(this, aRequest);
+  nsContentUtils::AddScriptRunner(runnable);
+}
+
+nsresult ScriptLoader::RestartLoad(ScriptLoadRequest* aRequest) {
+  aRequest->DropBytecode();
+  TRACE_FOR_TEST(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                 "scriptloader_fallback");
+
+  // Notify preload restart so that we can register this preload request again.
+  aRequest->GetScriptLoadContext()->NotifyRestart(mDocument);
+
+  // Start a new channel from which we explicitly request to stream the source
+  // instead of the bytecode.
+  aRequest->mFetchSourceOnly = true;
+  nsresult rv;
+  if (aRequest->IsModuleRequest()) {
+    rv = aRequest->AsModuleRequest()->RestartModuleLoad();
+  } else {
+    rv = StartLoad(aRequest, Nothing());
+  }
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // Close the current channel and this ScriptLoadHandler as we created a new
+  // one for the same request.
+  return NS_BINDING_RETARGETED;
+}
+
+nsresult ScriptLoader::StartLoad(
+    ScriptLoadRequest* aRequest,
+    const Maybe<nsAutoString>& aCharsetForPreload) {
+  if (aRequest->IsModuleRequest()) {
+    return aRequest->AsModuleRequest()->StartModuleLoad();
+  }
+
+  return StartClassicLoad(aRequest, aCharsetForPreload);
+}
+
+nsresult ScriptLoader::StartClassicLoad(
+    ScriptLoadRequest* aRequest,
+    const Maybe<nsAutoString>& aCharsetForPreload) {
+  MOZ_ASSERT(aRequest->IsFetching());
+  NS_ENSURE_TRUE(mDocument, NS_ERROR_NULL_POINTER);
+  aRequest->SetUnknownDataType();
+
+  // If this document is sandboxed without 'allow-scripts', abort.
+  if (mDocument->HasScriptsBlockedBySandbox()) {
+    return NS_OK;
+  }
+
+  if (LOG_ENABLED()) {
+    nsAutoCString url;
+    aRequest->mURI->GetAsciiSpec(url);
+    LOG(("ScriptLoadRequest (%p): Start Classic Load (url = %s)", aRequest,
+         url.get()));
+  }
+
+  nsSecurityFlags securityFlags =
+      nsContentSecurityManager::ComputeSecurityFlags(
+          aRequest->CORSMode(), nsContentSecurityManager::CORSSecurityMapping::
+                                    CORS_NONE_MAPS_TO_DISABLED_CORS_CHECKS);
+
+  securityFlags |= nsILoadInfo::SEC_ALLOW_CHROME;
+
+  nsresult rv = StartLoadInternal(aRequest, securityFlags, aCharsetForPreload);
+
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
+}
+
+static bool IsWebExtensionRequest(ScriptLoadRequest* aRequest) {
+  if (!aRequest->IsModuleRequest()) {
+    return false;
+  }
+
+  ModuleLoader* loader =
+      ModuleLoader::From(aRequest->AsModuleRequest()->mLoader);
+  return loader->GetKind() == ModuleLoader::WebExtension;
+}
+
+static nsresult CreateChannelForScriptLoading(nsIChannel** aOutChannel,
+                                              Document* aDocument,
+                                              ScriptLoadRequest* aRequest,
+                                              nsSecurityFlags aSecurityFlags) {
+  nsContentPolicyType contentPolicyType =
+      ScriptLoadRequestToContentPolicyType(aRequest);
+  nsCOMPtr<nsINode> context;
+  if (aRequest->GetScriptLoadContext()->GetScriptElement()) {
+    context =
+        do_QueryInterface(aRequest->GetScriptLoadContext()->GetScriptElement());
+  } else {
+    context = aDocument;
+  }
+
+  nsCOMPtr<nsILoadGroup> loadGroup = aDocument->GetDocumentLoadGroup();
+  nsCOMPtr<nsPIDOMWindowOuter> window = aDocument->GetWindow();
+  NS_ENSURE_TRUE(window, NS_ERROR_NULL_POINTER);
+  nsIDocShell* docshell = window->GetDocShell();
+  nsCOMPtr<nsIInterfaceRequestor> prompter(do_QueryInterface(docshell));
+
+  return NS_NewChannelWithTriggeringPrincipal(
+      aOutChannel, aRequest->mURI, context, aRequest->TriggeringPrincipal(),
+      aSecurityFlags, contentPolicyType,
+      nullptr,  // aPerformanceStorage
+      loadGroup, prompter);
+}
+
+static void PrepareLoadInfoForScriptLoading(nsIChannel* aChannel,
+                                            const ScriptLoadRequest* aRequest) {
+  nsCOMPtr<nsILoadInfo> loadInfo = aChannel->LoadInfo();
+  loadInfo->SetParserCreatedScript(aRequest->ParserMetadata() ==
+                                   ParserMetadata::ParserInserted);
+  loadInfo->SetCspNonce(aRequest->Nonce());
+  if (aRequest->mIntegrity.IsValid()) {
+    MOZ_ASSERT(!aRequest->mIntegrity.IsEmpty());
+    loadInfo->SetIntegrityMetadata(aRequest->mIntegrity.GetIntegrityString());
+  }
+}
+
+// static
+void ScriptLoader::PrepareCacheInfoChannel(nsIChannel* aChannel,
+                                           ScriptLoadRequest* aRequest) {
+  // To avoid decoding issues, the build-id is part of the bytecode MIME type
+  // constant.
+  aRequest->mCacheInfo = nullptr;
+  nsCOMPtr<nsICacheInfoChannel> cic(do_QueryInterface(aChannel));
+  if (cic && StaticPrefs::dom_script_loader_bytecode_cache_enabled()) {
+    MOZ_ASSERT(!IsWebExtensionRequest(aRequest),
+               "Can not bytecode cache WebExt code");
+    if (!aRequest->mFetchSourceOnly) {
+      // Inform the HTTP cache that we prefer to have information coming from
+      // the bytecode cache instead of the sources, if such entry is already
+      // registered.
+      LOG(("ScriptLoadRequest (%p): Maybe request bytecode", aRequest));
+      cic->PreferAlternativeDataType(
+          ScriptLoader::BytecodeMimeTypeFor(aRequest), ""_ns,
+          nsICacheInfoChannel::PreferredAlternativeDataDeliveryType::ASYNC);
+    } else {
+      // If we are explicitly loading from the sources, such as after a
+      // restarted request, we might still want to save the bytecode after.
+      //
+      // The following tell the cache to look for an alternative data type which
+      // does not exist, such that we can later save the bytecode with a
+      // different alternative data type.
+      LOG(("ScriptLoadRequest (%p): Request saving bytecode later", aRequest));
+      cic->PreferAlternativeDataType(
+          kNullMimeType, ""_ns,
+          nsICacheInfoChannel::PreferredAlternativeDataDeliveryType::ASYNC);
+    }
+  }
+}
+
+static void AdjustPriorityAndClassOfServiceForLinkPreloadScripts(
+    nsIChannel* aChannel, ScriptLoadRequest* aRequest) {
+  MOZ_ASSERT(aRequest->GetScriptLoadContext()->IsLinkPreloadScript());
+
+  if (!StaticPrefs::network_fetchpriority_enabled()) {
+    // Put it to the group that is not blocked by leaders and doesn't block
+    // follower at the same time.
+    // Giving it a much higher priority will make this request be processed
+    // ahead of other Unblocked requests, but with the same weight as
+    // Leaders. This will make us behave similar way for both http2 and http1.
+    ScriptLoadContext::PrioritizeAsPreload(aChannel);
+    return;
+  }
+
+  if (nsCOMPtr<nsIClassOfService> cos = do_QueryInterface(aChannel)) {
+    cos->AddClassFlags(nsIClassOfService::Unblocked);
+  }
+
+  if (nsCOMPtr<nsISupportsPriority> supportsPriority =
+          do_QueryInterface(aChannel)) {
+    LOG(("Is <link rel=[module]preload"));
+
+    const RequestPriority fetchPriority = aRequest->FetchPriority();
+    // The spec defines the priority to be set in an implementation defined
+    // manner (<https://fetch.spec.whatwg.org/#concept-fetch>, step 15 and
+    // <https://html.spec.whatwg.org/#concept-script-fetch-options-fetch-priority>).
+    // For web-compatibility, the fetch priority mapping from
+    // <https://web.dev/articles/fetch-priority#browser_priority_and_fetchpriority>
+    // is taken.
+    const int32_t supportsPriorityValue = [&]() {
+      switch (fetchPriority) {
+        case RequestPriority::Auto: {
+          return nsISupportsPriority::PRIORITY_HIGH;
+        }
+        case RequestPriority::High: {
+          return nsISupportsPriority::PRIORITY_HIGH;
+        }
+        case RequestPriority::Low: {
+          return nsISupportsPriority::PRIORITY_LOW;
+        }
+        default: {
+          MOZ_ASSERT_UNREACHABLE();
+          return nsISupportsPriority::PRIORITY_NORMAL;
+        }
+      }
+    }();
+
+    LogPriorityMapping(ScriptLoader::gScriptLoaderLog,
+                       ToFetchPriority(fetchPriority), supportsPriorityValue);
+
+    supportsPriority->SetPriority(supportsPriorityValue);
+  }
+}
+
+void AdjustPriorityForNonLinkPreloadScripts(nsIChannel* aChannel,
+                                            ScriptLoadRequest* aRequest) {
+  MOZ_ASSERT(!aRequest->GetScriptLoadContext()->IsLinkPreloadScript());
+
+  if (!StaticPrefs::network_fetchpriority_enabled()) {
+    return;
+  }
+
+  if (nsCOMPtr<nsISupportsPriority> supportsPriority =
+          do_QueryInterface(aChannel)) {
+    LOG(("Is not <link rel=[module]preload"));
+    const RequestPriority fetchPriority = aRequest->FetchPriority();
+
+    // The spec defines the priority to be set in an implementation defined
+    // manner (<https://fetch.spec.whatwg.org/#concept-fetch>, step 15 and
+    // <https://html.spec.whatwg.org/#concept-script-fetch-options-fetch-priority>).
+    // <testing/web-platform/mozilla/tests/fetch/fetchpriority/support/script-tests-data.js>
+    // provides more context for the priority mapping.
+    const int32_t supportsPriorityValue = [&]() {
+      switch (fetchPriority) {
+        case RequestPriority::Auto: {
+          if (aRequest->IsModuleRequest()) {
+            return nsISupportsPriority::PRIORITY_HIGH;
+          }
+
+          const ScriptLoadContext* scriptLoadContext =
+              aRequest->GetScriptLoadContext();
+          if (scriptLoadContext->IsAsyncScript() ||
+              scriptLoadContext->IsDeferredScript()) {
+            return nsISupportsPriority::PRIORITY_LOW;
+          }
+
+          if (scriptLoadContext->mScriptFromHead) {
+            return nsISupportsPriority::PRIORITY_HIGH;
+          }
+
+          return nsISupportsPriority::PRIORITY_NORMAL;
+        }
+        case RequestPriority::Low: {
+          return nsISupportsPriority::PRIORITY_LOW;
+        }
+        case RequestPriority::High: {
+          return nsISupportsPriority::PRIORITY_HIGH;
+        }
+        default: {
+          MOZ_ASSERT_UNREACHABLE();
+          return nsISupportsPriority::PRIORITY_NORMAL;
+        }
+      }
+    }();
+
+    if (supportsPriorityValue) {
+      LogPriorityMapping(ScriptLoader::gScriptLoaderLog,
+                         ToFetchPriority(fetchPriority), supportsPriorityValue);
+      supportsPriority->SetPriority(supportsPriorityValue);
+    }
+  }
+}
+
+// static
+void ScriptLoader::PrepareRequestPriorityAndRequestDependencies(
+    nsIChannel* aChannel, ScriptLoadRequest* aRequest) {
+  if (aRequest->GetScriptLoadContext()->IsLinkPreloadScript()) {
+    // This is <link rel="preload" as="script"> or <link rel="modulepreload">
+    // initiated speculative load
+    // (https://developer.mozilla.org/en-US/docs/Web/Performance/Speculative_loading).
+    AdjustPriorityAndClassOfServiceForLinkPreloadScripts(aChannel, aRequest);
+    ScriptLoadContext::AddLoadBackgroundFlag(aChannel);
+  } else if (nsCOMPtr<nsIClassOfService> cos = do_QueryInterface(aChannel)) {
+    AdjustPriorityForNonLinkPreloadScripts(aChannel, aRequest);
+
+    if (aRequest->GetScriptLoadContext()->mScriptFromHead &&
+        aRequest->GetScriptLoadContext()->IsBlockingScript()) {
+      // synchronous head scripts block loading of most other non js/css
+      // content such as images, Leader implicitely disallows tailing
+      cos->AddClassFlags(nsIClassOfService::Leader);
+    } else if (aRequest->GetScriptLoadContext()->IsDeferredScript() &&
+               !StaticPrefs::network_http_tailing_enabled()) {
+      // Bug 1395525 and the !StaticPrefs::network_http_tailing_enabled() bit:
+      // We want to make sure that turing tailing off by the pref makes the
+      // browser behave exactly the same way as before landing the tailing
+      // patch.
+
+      // head/body deferred scripts are blocked by leaders but are not
+      // allowed tailing because they block DOMContentLoaded
+      cos->AddClassFlags(nsIClassOfService::TailForbidden);
+    } else {
+      // other scripts (=body sync or head/body async) are neither blocked
+      // nor prioritized
+      cos->AddClassFlags(nsIClassOfService::Unblocked);
+
+      if (aRequest->GetScriptLoadContext()->IsAsyncScript()) {
+        // async scripts are allowed tailing, since those and only those
+        // don't block DOMContentLoaded; this flag doesn't enforce tailing,
+        // just overweights the Unblocked flag when the channel is found
+        // to be a thrird-party tracker and thus set the Tail flag to engage
+        // tailing.
+        cos->AddClassFlags(nsIClassOfService::TailAllowed);
+      }
+    }
+  }
+}
+
+// static
+nsresult ScriptLoader::PrepareHttpRequestAndInitiatorType(
+    nsIChannel* aChannel, ScriptLoadRequest* aRequest,
+    const Maybe<nsAutoString>& aCharsetForPreload) {
+  nsCOMPtr<nsIHttpChannel> httpChannel(do_QueryInterface(aChannel));
+  nsresult rv = NS_OK;
+
+  if (httpChannel) {
+    // HTTP content negotation has little value in this context.
+    nsAutoCString acceptTypes("*/*");
+    rv = httpChannel->SetRequestHeader("Accept"_ns, acceptTypes, false);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+
+    nsCOMPtr<nsIReferrerInfo> referrerInfo =
+        new ReferrerInfo(aRequest->mReferrer, aRequest->ReferrerPolicy());
+    rv = httpChannel->SetReferrerInfoWithoutClone(referrerInfo);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+
+    nsCOMPtr<nsIHttpChannelInternal> internalChannel(
+        do_QueryInterface(httpChannel));
+    if (internalChannel) {
+      rv = internalChannel->SetIntegrityMetadata(
+          aRequest->mIntegrity.GetIntegrityString());
+      MOZ_ASSERT(NS_SUCCEEDED(rv));
+    }
+
+    nsAutoString hintCharset;
+    if (!aRequest->GetScriptLoadContext()->IsPreload() &&
+        aRequest->GetScriptLoadContext()->GetScriptElement()) {
+      aRequest->GetScriptLoadContext()->GetScriptElement()->GetScriptCharset(
+          hintCharset);
+    } else if (aCharsetForPreload.isSome()) {
+      hintCharset = aCharsetForPreload.ref();
+    }
+
+    rv = httpChannel->SetClassicScriptHintCharset(hintCharset);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  // Set the initiator type
+  nsCOMPtr<nsITimedChannel> timedChannel(do_QueryInterface(httpChannel));
+  if (timedChannel) {
+    if (aRequest->mEarlyHintPreloaderId) {
+      timedChannel->SetInitiatorType(u"early-hints"_ns);
+    } else if (aRequest->GetScriptLoadContext()->IsLinkPreloadScript()) {
+      timedChannel->SetInitiatorType(u"link"_ns);
+    } else {
+      timedChannel->SetInitiatorType(u"script"_ns);
+    }
+  }
+
+  return rv;
+}
+
+nsresult ScriptLoader::PrepareIncrementalStreamLoader(
+    nsIIncrementalStreamLoader** aOutLoader, ScriptLoadRequest* aRequest) {
+  UniquePtr<mozilla::dom::SRICheckDataVerifier> sriDataVerifier;
+  if (!aRequest->mIntegrity.IsEmpty()) {
+    nsAutoCString sourceUri;
+    if (mDocument->GetDocumentURI()) {
+      mDocument->GetDocumentURI()->GetAsciiSpec(sourceUri);
+    }
+    sriDataVerifier = MakeUnique<SRICheckDataVerifier>(aRequest->mIntegrity,
+                                                       sourceUri, mReporter);
+  }
+
+  RefPtr<ScriptLoadHandler> handler =
+      new ScriptLoadHandler(this, aRequest, std::move(sriDataVerifier));
+
+  nsresult rv = NS_NewIncrementalStreamLoader(aOutLoader, handler);
+  NS_ENSURE_SUCCESS(rv, rv);
+  return rv;
+}
+
+nsresult ScriptLoader::StartLoadInternal(
+    ScriptLoadRequest* aRequest, nsSecurityFlags securityFlags,
+    const Maybe<nsAutoString>& aCharsetForPreload) {
+  nsCOMPtr<nsIChannel> channel;
+  nsresult rv = CreateChannelForScriptLoading(
+      getter_AddRefs(channel), mDocument, aRequest, securityFlags);
+
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (aRequest->mEarlyHintPreloaderId) {
+    nsCOMPtr<nsIHttpChannelInternal> channelInternal =
+        do_QueryInterface(channel);
+    NS_ENSURE_TRUE(channelInternal != nullptr, NS_ERROR_FAILURE);
+
+    rv = channelInternal->SetEarlyHintPreloaderId(
+        aRequest->mEarlyHintPreloaderId);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  PrepareLoadInfoForScriptLoading(channel, aRequest);
+
+  nsCOMPtr<nsIScriptGlobalObject> scriptGlobal = GetScriptGlobalObject();
+  if (!scriptGlobal) {
+    return NS_ERROR_FAILURE;
+  }
+
+  ScriptLoader::PrepareCacheInfoChannel(channel, aRequest);
+
+  LOG(("ScriptLoadRequest (%p): mode=%u tracking=%d", aRequest,
+       unsigned(aRequest->GetScriptLoadContext()->mScriptMode),
+       aRequest->GetScriptLoadContext()->IsTracking()));
+
+  PrepareRequestPriorityAndRequestDependencies(channel, aRequest);
+
+  rv =
+      PrepareHttpRequestAndInitiatorType(channel, aRequest, aCharsetForPreload);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  mozilla::net::PredictorLearn(
+      aRequest->mURI, mDocument->GetDocumentURI(),
+      nsINetworkPredictor::LEARN_LOAD_SUBRESOURCE,
+      mDocument->NodePrincipal()->OriginAttributesRef());
+
+  nsCOMPtr<nsIIncrementalStreamLoader> loader;
+  rv = PrepareIncrementalStreamLoader(getter_AddRefs(loader), aRequest);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  auto key = PreloadHashKey::CreateAsScript(
+      aRequest->mURI, aRequest->CORSMode(), aRequest->mKind);
+  aRequest->GetScriptLoadContext()->NotifyOpen(
+      key, channel, mDocument,
+      aRequest->GetScriptLoadContext()->IsLinkPreloadScript(),
+      aRequest->IsModuleRequest());
+
+  rv = channel->AsyncOpen(loader);
+
+  if (NS_FAILED(rv)) {
+    // Make sure to inform any <link preload> tags about failure to load the
+    // resource.
+    aRequest->GetScriptLoadContext()->NotifyStart(channel);
+    aRequest->GetScriptLoadContext()->NotifyStop(rv);
+  }
+
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
+}
+
+bool ScriptLoader::PreloadURIComparator::Equals(const PreloadInfo& aPi,
+                                                nsIURI* const& aURI) const {
+  bool same;
+  return NS_SUCCEEDED(aPi.mRequest->mURI->Equals(aURI, &same)) && same;
+}
+
+static bool CSPAllowsInlineScript(nsIScriptElement* aElement,
+                                  const nsAString& aNonce,
+                                  Document* aDocument) {
+  nsCOMPtr<nsIContentSecurityPolicy> csp = aDocument->GetCsp();
+  if (!csp) {
+    // no CSP --> allow
+    return true;
+  }
+
+  bool parserCreated =
+      aElement->GetParserCreated() != mozilla::dom::NOT_FROM_PARSER;
+  nsCOMPtr<Element> element = do_QueryInterface(aElement);
+
+  bool allowInlineScript = false;
+  nsresult rv = csp->GetAllowsInline(
+      nsIContentSecurityPolicy::SCRIPT_SRC_ELEM_DIRECTIVE,
+      false /* aHasUnsafeHash */, aNonce, parserCreated, element,
+      nullptr /* nsICSPEventListener */, u""_ns,
+      aElement->GetScriptLineNumber(),
+      aElement->GetScriptColumnNumber().oneOriginValue(), &allowInlineScript);
+  return NS_SUCCEEDED(rv) && allowInlineScript;
+}
+
+namespace {
+RequestPriority FetchPriorityToRequestPriority(
+    const FetchPriority aFetchPriority) {
+  switch (aFetchPriority) {
+    case FetchPriority::High:
+      return RequestPriority::High;
+    case FetchPriority::Low:
+      return RequestPriority::Low;
+    case FetchPriority::Auto:
+      return RequestPriority::Auto;
+  }
+
+  MOZ_ASSERT_UNREACHABLE();
+  return RequestPriority::Auto;
+}
+}  // namespace
+
+already_AddRefed<ScriptLoadRequest> ScriptLoader::CreateLoadRequest(
+    ScriptKind aKind, nsIURI* aURI, nsIScriptElement* aElement,
+    nsIPrincipal* aTriggeringPrincipal, CORSMode aCORSMode,
+    const nsAString& aNonce, RequestPriority aRequestPriority,
+    const SRIMetadata& aIntegrity, ReferrerPolicy aReferrerPolicy,
+    ParserMetadata aParserMetadata) {
+  nsIURI* referrer = mDocument->GetDocumentURIAsReferrer();
+  nsCOMPtr<Element> domElement = do_QueryInterface(aElement);
+  RefPtr<ScriptFetchOptions> fetchOptions =
+      new ScriptFetchOptions(aCORSMode, aNonce, aRequestPriority,
+                             aParserMetadata, aTriggeringPrincipal, domElement);
+  RefPtr<ScriptLoadContext> context = new ScriptLoadContext();
+
+  if (aKind == ScriptKind::eClassic || aKind == ScriptKind::eImportMap) {
+    RefPtr<ScriptLoadRequest> aRequest =
+        new ScriptLoadRequest(aKind, aURI, aReferrerPolicy, fetchOptions,
+                              aIntegrity, referrer, context);
+
+    aRequest->NoCacheEntryFound();
+    return aRequest.forget();
+  }
+
+  MOZ_ASSERT(aKind == ScriptKind::eModule);
+  RefPtr<ModuleLoadRequest> aRequest = ModuleLoader::CreateTopLevel(
+      aURI, aReferrerPolicy, fetchOptions, aIntegrity, referrer, this, context);
+  return aRequest.forget();
+}
+
+bool ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement) {
+  // We need a document to evaluate scripts.
+  NS_ENSURE_TRUE(mDocument, false);
+
+  // Check to see if scripts has been turned off.
+  if (!mEnabled || !mDocument->IsScriptEnabled()) {
+    return false;
+  }
+
+  NS_ASSERTION(!aElement->IsMalformed(), "Executing malformed script");
+
+  nsCOMPtr<nsIContent> scriptContent = do_QueryInterface(aElement);
+
+  ScriptKind scriptKind;
+  if (aElement->GetScriptIsModule()) {
+    scriptKind = ScriptKind::eModule;
+  } else if (aElement->GetScriptIsImportMap()) {
+    scriptKind = ScriptKind::eImportMap;
+  } else {
+    scriptKind = ScriptKind::eClassic;
+  }
+
+  // Step 13. Check that the script is not an eventhandler
+  if (IsScriptEventHandler(scriptKind, scriptContent)) {
+    return false;
+  }
+
+  // "In modern user agents that support module scripts, the script element with
+  // the nomodule attribute will be ignored".
+  // "The nomodule attribute must not be specified on module scripts (and will
+  // be ignored if it is)."
+  if (scriptKind == ScriptKind::eClassic && scriptContent->IsHTMLElement() &&
+      scriptContent->AsElement()->HasAttr(nsGkAtoms::nomodule)) {
+    return false;
+  }
+
+  // Step 15. and later in the HTML5 spec
+  if (aElement->GetScriptExternal()) {
+    return ProcessExternalScript(aElement, scriptKind, scriptContent);
+  }
+
+  return ProcessInlineScript(aElement, scriptKind);
+}
+
+static ParserMetadata GetParserMetadata(nsIScriptElement* aElement) {
+  return aElement->GetParserCreated() == mozilla::dom::NOT_FROM_PARSER
+             ? ParserMetadata::NotParserInserted
+             : ParserMetadata::ParserInserted;
+}
+
+bool ScriptLoader::ProcessExternalScript(nsIScriptElement* aElement,
+                                         ScriptKind aScriptKind,
+                                         nsIContent* aScriptContent) {
+  LOG(("ScriptLoader (%p): Process external script for element %p", this,
+       aElement));
+
+  // https://html.spec.whatwg.org/multipage/scripting.html#prepare-the-script-element
+  // Step 30.1. If el's type is "importmap", then queue an element task on the
+  // DOM manipulation task source given el to fire an event named error at el,
+  // and return.
+  if (aScriptKind == ScriptKind::eImportMap) {
+    NS_DispatchToCurrentThread(
+        NewRunnableMethod("nsIScriptElement::FireErrorEvent", aElement,
+                          &nsIScriptElement::FireErrorEvent));
+    nsContentUtils::ReportToConsole(
+        nsIScriptError::warningFlag, "Script Loader"_ns, mDocument,
+        nsContentUtils::eDOM_PROPERTIES, "ImportMapExternalNotSupported");
+    return false;
+  }
+
+  nsCOMPtr<nsIURI> scriptURI = aElement->GetScriptURI();
+  if (!scriptURI) {
+    // Asynchronously report the failure to create a URI object
+    NS_DispatchToCurrentThread(
+        NewRunnableMethod("nsIScriptElement::FireErrorEvent", aElement,
+                          &nsIScriptElement::FireErrorEvent));
+    return false;
+  }
+
+  nsString nonce = nsContentSecurityUtils::GetIsElementNonceableNonce(
+      *aScriptContent->AsElement());
+  SRIMetadata sriMetadata;
+  {
+    nsAutoString integrity;
+    aScriptContent->AsElement()->GetAttr(nsGkAtoms::integrity, integrity);
+    GetSRIMetadata(integrity, &sriMetadata);
+  }
+
+  RefPtr<ScriptLoadRequest> request =
+      LookupPreloadRequest(aElement, aScriptKind, sriMetadata);
+
+  if (request &&
+      NS_FAILED(CheckContentPolicy(mDocument, aElement, nonce, request))) {
+    LOG(("ScriptLoader (%p): content policy check failed for preload", this));
+
+    // Probably plans have changed; even though the preload was allowed seems
+    // like the actual load is not; let's cancel the preload request.
+    request->Cancel();
+    AccumulateCategorical(LABELS_DOM_SCRIPT_PRELOAD_RESULT::RejectedByPolicy);
+    return false;
+  }
+
+  if (request && request->IsModuleRequest() &&
+      mModuleLoader->HasImportMapRegistered() &&
+      request->mState > ScriptLoadRequest::State::Compiling) {
+    // We don't preload module scripts after seeing an import map but a script
+    // can dynamically insert an import map after preloading has happened.
+    //
+    // In the case of an import map is inserted after preloading has happened,
+    // We also check if the request has started loading imports, if not then we
+    // can reuse the preloaded request.
+    request->Cancel();
+    request = nullptr;
+  }
+
+  if (request) {
+    // Use the preload request.
+
+    LOG(("ScriptLoadRequest (%p): Using preload request", request.get()));
+
+    // https://html.spec.whatwg.org/multipage/webappapis.html#fetch-a-module-script-tree
+    // Step 1. Disallow further import maps given settings object.
+    if (request->IsModuleRequest()) {
+      LOG(("ScriptLoadRequest (%p): Disallow further import maps.",
+           request.get()));
+      mModuleLoader->DisallowImportMaps();
+    }
+
+    // It's possible these attributes changed since we started the preload so
+    // update them here.
+    request->GetScriptLoadContext()->SetScriptMode(
+        aElement->GetScriptDeferred(), aElement->GetScriptAsync(), false);
+
+    // The request will be added to another list or set as
+    // mParserBlockingRequest below.
+    if (request->GetScriptLoadContext()->mInCompilingList) {
+      mOffThreadCompilingRequests.Remove(request);
+      request->GetScriptLoadContext()->mInCompilingList = false;
+    }
+
+    AccumulateCategorical(LABELS_DOM_SCRIPT_PRELOAD_RESULT::Used);
+  } else {
+    // No usable preload found.
+
+    nsCOMPtr<nsIPrincipal> principal =
+        aElement->GetScriptURITriggeringPrincipal();
+    if (!principal) {
+      principal = aScriptContent->NodePrincipal();
+    }
+
+    CORSMode ourCORSMode = aElement->GetCORSMode();
+    const FetchPriority fetchPriority = aElement->GetFetchPriority();
+    ReferrerPolicy referrerPolicy = GetReferrerPolicy(aElement);
+    ParserMetadata parserMetadata = GetParserMetadata(aElement);
+
+    request = CreateLoadRequest(aScriptKind, scriptURI, aElement, principal,
+                                ourCORSMode, nonce,
+                                FetchPriorityToRequestPriority(fetchPriority),
+                                sriMetadata, referrerPolicy, parserMetadata);
+    request->GetScriptLoadContext()->mIsInline = false;
+    request->GetScriptLoadContext()->SetScriptMode(
+        aElement->GetScriptDeferred(), aElement->GetScriptAsync(), false);
+    // keep request->GetScriptLoadContext()->mScriptFromHead to false so we
+    // don't treat non preloaded scripts as blockers for full page load. See bug
+    // 792438.
+
+    LOG(("ScriptLoadRequest (%p): Created request for external script",
+         request.get()));
+
+    nsresult rv = StartLoad(request, Nothing());
+    if (NS_FAILED(rv)) {
+      ReportErrorToConsole(request, rv);
+
+      // Asynchronously report the load failure
+      nsCOMPtr<nsIRunnable> runnable =
+          NewRunnableMethod("nsIScriptElement::FireErrorEvent", aElement,
+                            &nsIScriptElement::FireErrorEvent);
+      if (mDocument) {
+        mDocument->Dispatch(runnable.forget());
+      } else {
+        NS_DispatchToCurrentThread(runnable.forget());
+      }
+      return false;
+    }
+  }
+
+  // We should still be in loading stage of script unless we're loading a
+  // module or speculatively off-main-thread parsing a script.
+  NS_ASSERTION(SpeculativeOMTParsingEnabled() ||
+                   !request->GetScriptLoadContext()->CompileStarted() ||
+                   request->IsModuleRequest(),
+               "Request should not yet be in compiling stage.");
+
+  if (request->GetScriptLoadContext()->IsAsyncScript()) {
+    AddAsyncRequest(request);
+    if (request->IsFinished()) {
+      // The script is available already. Run it ASAP when the event
+      // loop gets a chance to spin.
+
+      // KVKV TODO: Instead of processing immediately, try off-thread-parsing
+      // it and only schedule a pending ProcessRequest if that fails.
+      ProcessPendingRequestsAsync();
+    }
+    return false;
+  }
+  if (!aElement->GetParserCreated()) {
+    // Violate the HTML5 spec in order to make LABjs and the "order" plug-in
+    // for RequireJS work with their Gecko-sniffed code path. See
+    // http://lists.w3.org/Archives/Public/public-html/2010Oct/0088.html
+    request->GetScriptLoadContext()->mIsNonAsyncScriptInserted = true;
+    mNonAsyncExternalScriptInsertedRequests.AppendElement(request);
+    if (request->IsFinished()) {
+      // The script is available already. Run it ASAP when the event
+      // loop gets a chance to spin.
+      ProcessPendingRequestsAsync();
+    }
+    return false;
+  }
+  // we now have a parser-inserted request that may or may not be still
+  // loading
+  if (request->GetScriptLoadContext()->IsDeferredScript()) {
+    // We don't want to run this yet.
+    // If we come here, the script is a parser-created script and it has
+    // the defer attribute but not the async attribute OR it is a module
+    // script without the async attribute. Since a
+    // a parser-inserted script is being run, we came here by the parser
+    // running the script, which means the parser is still alive and the
+    // parse is ongoing.
+    NS_ASSERTION(mDocument->GetCurrentContentSink() ||
+                     aElement->GetParserCreated() == FROM_PARSER_XSLT,
+                 "Non-XSLT Defer script on a document without an active "
+                 "parser; bug 592366.");
+    AddDeferRequest(request);
+    return false;
+  }
+
+  if (aElement->GetParserCreated() == FROM_PARSER_XSLT) {
+    // Need to maintain order for XSLT-inserted scripts
+    NS_ASSERTION(!mParserBlockingRequest,
+                 "Parser-blocking scripts and XSLT scripts in the same doc!");
+    request->GetScriptLoadContext()->mIsXSLT = true;
+    mXSLTRequests.AppendElement(request);
+    if (request->IsFinished()) {
+      // The script is available already. Run it ASAP when the event
+      // loop gets a chance to spin.
+      ProcessPendingRequestsAsync();
+    }
+    return true;
+  }
+
+  if (request->IsFinished() && ReadyToExecuteParserBlockingScripts()) {
+    // The request has already been loaded and there are no pending style
+    // sheets. If the script comes from the network stream, cheat for
+    // performance reasons and avoid a trip through the event loop.
+    if (aElement->GetParserCreated() == FROM_PARSER_NETWORK) {
+      return ProcessRequest(request) == NS_ERROR_HTMLPARSER_BLOCK;
+    }
+    // Otherwise, we've got a document.written script, make a trip through
+    // the event loop to hide the preload effects from the scripts on the
+    // Web page.
+    NS_ASSERTION(!mParserBlockingRequest,
+                 "There can be only one parser-blocking script at a time");
+    NS_ASSERTION(mXSLTRequests.isEmpty(),
+                 "Parser-blocking scripts and XSLT scripts in the same doc!");
+    mParserBlockingRequest = request;
+    ProcessPendingRequestsAsync();
+    return true;
+  }
+
+  // The script hasn't loaded yet or there's a style sheet blocking it.
+  // The script will be run when it loads or the style sheet loads.
+  NS_ASSERTION(!mParserBlockingRequest,
+               "There can be only one parser-blocking script at a time");
+  NS_ASSERTION(mXSLTRequests.isEmpty(),
+               "Parser-blocking scripts and XSLT scripts in the same doc!");
+  mParserBlockingRequest = request;
+  return true;
+}
+
+bool ScriptLoader::ProcessInlineScript(nsIScriptElement* aElement,
+                                       ScriptKind aScriptKind) {
+  // Is this document sandboxed without 'allow-scripts'?
+  if (mDocument->HasScriptsBlockedBySandbox()) {
+    return false;
+  }
+
+  nsCOMPtr<Element> element = do_QueryInterface(aElement);
+  nsString nonce = nsContentSecurityUtils::GetIsElementNonceableNonce(*element);
+
+  // Does CSP allow this inline script to run?
+  if (!CSPAllowsInlineScript(aElement, nonce, mDocument)) {
+    return false;
+  }
+
+  // Check if adding an import map script is allowed. If not, we bail out
+  // early to prevent creating a load request.
+  if (aScriptKind == ScriptKind::eImportMap) {
+    // https://html.spec.whatwg.org/multipage/scripting.html#prepare-the-script-element
+    // Step 31.2 type is "importmap":
+    //   Step 1. If el's relevant global object's import maps allowed is false,
+    //   then queue an element task on the DOM manipulation task source given el
+    //   to fire an event named error at el, and return.
+    if (!mModuleLoader->IsImportMapAllowed()) {
+      NS_WARNING("ScriptLoader: import maps allowed is false.");
+      const char* msg = mModuleLoader->HasImportMapRegistered()
+                            ? "ImportMapNotAllowedMultiple"
+                            : "ImportMapNotAllowedAfterModuleLoad";
+      nsContentUtils::ReportToConsole(nsIScriptError::warningFlag,
+                                      "Script Loader"_ns, mDocument,
+                                      nsContentUtils::eDOM_PROPERTIES, msg);
+      NS_DispatchToCurrentThread(
+          NewRunnableMethod("nsIScriptElement::FireErrorEvent", aElement,
+                            &nsIScriptElement::FireErrorEvent));
+      return false;
+    }
+  }
+
+  // Inline classic scripts ignore their CORS mode and are always CORS_NONE.
+  CORSMode corsMode = CORS_NONE;
+  if (aScriptKind == ScriptKind::eModule) {
+    corsMode = aElement->GetCORSMode();
+  }
+  // <https://html.spec.whatwg.org/multipage/scripting.html#prepare-the-script-element>
+  // step 29 specifies to use the fetch priority. Presumably it has no effect
+  // for inline scripts.
+  const auto fetchPriority = aElement->GetFetchPriority();
+
+  ReferrerPolicy referrerPolicy = GetReferrerPolicy(aElement);
+  ParserMetadata parserMetadata = GetParserMetadata(aElement);
+
+  // NOTE: The `nonce` as specified here is significant, because it's inherited
+  // by other scripts (e.g. modules created via dynamic imports).
+  RefPtr<ScriptLoadRequest> request =
+      CreateLoadRequest(aScriptKind, mDocument->GetDocumentURI(), aElement,
+                        mDocument->NodePrincipal(), corsMode, nonce,
+                        FetchPriorityToRequestPriority(fetchPriority),
+                        SRIMetadata(),  // SRI doesn't apply
+                        referrerPolicy, parserMetadata);
+  request->GetScriptLoadContext()->mIsInline = true;
+  request->GetScriptLoadContext()->mLineNo = aElement->GetScriptLineNumber();
+  request->GetScriptLoadContext()->mColumnNo =
+      aElement->GetScriptColumnNumber();
+  request->mFetchSourceOnly = true;
+  request->SetTextSource(request->mLoadContext.get());
+  TRACE_FOR_TEST_BOOL(request->GetScriptLoadContext()->GetScriptElement(),
+                      "scriptloader_load_source");
+  CollectScriptTelemetry(request);
+
+  // Only the 'async' attribute is heeded on an inline module script and
+  // inline classic scripts ignore both these attributes.
+  MOZ_ASSERT(!aElement->GetScriptDeferred());
+  MOZ_ASSERT_IF(!request->IsModuleRequest(), !aElement->GetScriptAsync());
+  request->GetScriptLoadContext()->SetScriptMode(
+      false, aElement->GetScriptAsync(), false);
+
+  LOG(("ScriptLoadRequest (%p): Created request for inline script",
+       request.get()));
+
+  request->mBaseURL = mDocument->GetDocBaseURI();
+
+  if (request->IsModuleRequest()) {
+    // https://html.spec.whatwg.org/multipage/webappapis.html#fetch-an-inline-module-script-graph
+    // Step 1. Disallow further import maps given settings object.
+    mModuleLoader->DisallowImportMaps();
+
+    ModuleLoadRequest* modReq = request->AsModuleRequest();
+    if (aElement->GetParserCreated() != NOT_FROM_PARSER) {
+      if (aElement->GetScriptAsync()) {
+        AddAsyncRequest(modReq);
+      } else {
+        AddDeferRequest(modReq);
+      }
+    }
+
+    // This calls OnFetchComplete directly since there's no need to start
+    // fetching an inline script.
+    nsresult rv = modReq->OnFetchComplete(NS_OK);
+    if (NS_FAILED(rv)) {
+      ReportErrorToConsole(modReq, rv);
+      HandleLoadError(modReq, rv);
+    }
+
+    return false;
+  }
+
+  if (request->IsImportMapRequest()) {
+    // https://html.spec.whatwg.org/multipage/scripting.html#prepare-the-script-element
+    // Step 31.2 type is "importmap":
+    //   Impl note: Step 1 is done above before creating a ScriptLoadRequest.
+    MOZ_ASSERT(mModuleLoader->IsImportMapAllowed());
+
+    //   Step 2. Set el's relevant global object's import maps allowed to false.
+    mModuleLoader->DisallowImportMaps();
+
+    //   Step 3. Let result be the result of creating an import map parse result
+    //   given source text and base URL.
+    UniquePtr<ImportMap> importMap = mModuleLoader->ParseImportMap(request);
+    if (!importMap) {
+      // If parsing import maps fails, the exception will be reported in
+      // ModuleLoaderBase::ParseImportMap, and the registration of the import
+      // map will bail out early.
+      return false;
+    }
+
+    // TODO: Bug 1781758: Move RegisterImportMap into EvaluateScriptElement.
+    //
+    // https://html.spec.whatwg.org/multipage/scripting.html#execute-the-script-element
+    // The spec defines 'register an import map' should be done in
+    // 'execute the script element', because inside 'execute the script element'
+    // it will perform a 'preparation-time document check'.
+    // However, as import maps could be only inline scripts by now, the
+    // 'preparation-time document check' will never fail for import maps.
+    // So we simply call 'register an import map' here.
+    mModuleLoader->RegisterImportMap(std::move(importMap));
+    return false;
+  }
+
+  request->mState = ScriptLoadRequest::State::Ready;
+  if (aElement->GetParserCreated() == FROM_PARSER_XSLT &&
+      (!ReadyToExecuteParserBlockingScripts() || !mXSLTRequests.isEmpty())) {
+    // Need to maintain order for XSLT-inserted scripts
+    NS_ASSERTION(!mParserBlockingRequest,
+                 "Parser-blocking scripts and XSLT scripts in the same doc!");
+    mXSLTRequests.AppendElement(request);
+    return true;
+  }
+  if (aElement->GetParserCreated() == NOT_FROM_PARSER) {
+    NS_ASSERTION(
+        !nsContentUtils::IsSafeToRunScript(),
+        "A script-inserted script is inserted without an update batch?");
+    RunScriptWhenSafe(request);
+    return false;
+  }
+  if (aElement->GetParserCreated() == FROM_PARSER_NETWORK &&
+      !ReadyToExecuteParserBlockingScripts()) {
+    NS_ASSERTION(!mParserBlockingRequest,
+                 "There can be only one parser-blocking script at a time");
+    mParserBlockingRequest = request;
+    NS_ASSERTION(mXSLTRequests.isEmpty(),
+                 "Parser-blocking scripts and XSLT scripts in the same doc!");
+    return true;
+  }
+  // We now have a document.written inline script or we have an inline script
+  // from the network but there is no style sheet that is blocking scripts.
+  // Don't check for style sheets blocking scripts in the document.write
+  // case to avoid style sheet network activity affecting when
+  // document.write returns. It's not really necessary to do this if
+  // there's no document.write currently on the call stack. However,
+  // this way matches IE more closely than checking if document.write
+  // is on the call stack.
+  NS_ASSERTION(nsContentUtils::IsSafeToRunScript(),
+               "Not safe to run a parser-inserted script?");
+  return ProcessRequest(request) == NS_ERROR_HTMLPARSER_BLOCK;
+}
+
+ScriptLoadRequest* ScriptLoader::LookupPreloadRequest(
+    nsIScriptElement* aElement, ScriptKind aScriptKind,
+    const SRIMetadata& aSRIMetadata) {
+  MOZ_ASSERT(aElement);
+
+  nsTArray<PreloadInfo>::index_type i =
+      mPreloads.IndexOf(aElement->GetScriptURI(), 0, PreloadURIComparator());
+  if (i == nsTArray<PreloadInfo>::NoIndex) {
+    return nullptr;
+  }
+  RefPtr<ScriptLoadRequest> request = mPreloads[i].mRequest;
+  if (aScriptKind != request->mKind) {
+    return nullptr;
+  }
+
+  // Found preloaded request. Note that a script-inserted script can steal a
+  // preload!
+  request->GetScriptLoadContext()->SetIsLoadRequest(aElement);
+
+  if (request->GetScriptLoadContext()->mWasCompiledOMT &&
+      !request->IsModuleRequest()) {
+    request->SetReady();
+  }
+
+  nsString preloadCharset(mPreloads[i].mCharset);
+  mPreloads.RemoveElementAt(i);
+
+  // Double-check that the charset the preload used is the same as the charset
+  // we have now.
+  nsAutoString elementCharset;
+  aElement->GetScriptCharset(elementCharset);
+
+  // Bug 1832361: charset and crossorigin attributes shouldn't affect matching
+  // of module scripts and modulepreload
+  if (!request->IsModuleRequest() &&
+      (!elementCharset.Equals(preloadCharset) ||
+       aElement->GetCORSMode() != request->CORSMode())) {
+    // Drop the preload.
+    request->Cancel();
+    AccumulateCategorical(LABELS_DOM_SCRIPT_PRELOAD_RESULT::RequestMismatch);
+    return nullptr;
+  }
+
+  if (!aSRIMetadata.CanTrustBeDelegatedTo(request->mIntegrity)) {
+    // Don't cancel link preload requests, we want to deliver onload according
+    // the result of the load, cancellation would unexpectedly lead to error
+    // notification.
+    if (!request->GetScriptLoadContext()->IsLinkPreloadScript()) {
+      request->Cancel();
+    }
+    return nullptr;
+  }
+
+  // Report any errors that we skipped while preloading.
+  ReportPreloadErrorsToConsole(request);
+
+  // This makes sure the pending preload (if exists) for this resource is
+  // properly marked as used and thus not notified in the console as unused.
+  request->GetScriptLoadContext()->NotifyUsage(mDocument);
+  // A used preload must no longer be found in the Document's hash table.  Any
+  // <link preload> tag after the <script> tag will start a new request, that
+  // can be satisfied from a different cache, but not from the preload cache.
+  request->GetScriptLoadContext()->RemoveSelf(mDocument);
+
+  return request;
+}
+
+void ScriptLoader::GetSRIMetadata(const nsAString& aIntegrityAttr,
+                                  SRIMetadata* aMetadataOut) {
+  MOZ_ASSERT(aMetadataOut->IsEmpty());
+
+  if (aIntegrityAttr.IsEmpty()) {
+    return;
+  }
+
+  MOZ_LOG(SRILogHelper::GetSriLog(), mozilla::LogLevel::Debug,
+          ("ScriptLoader::GetSRIMetadata, integrity=%s",
+           NS_ConvertUTF16toUTF8(aIntegrityAttr).get()));
+
+  nsAutoCString sourceUri;
+  if (mDocument->GetDocumentURI()) {
+    mDocument->GetDocumentURI()->GetAsciiSpec(sourceUri);
+  }
+  SRICheck::IntegrityMetadata(aIntegrityAttr, sourceUri, mReporter,
+                              aMetadataOut);
+}
+
+ReferrerPolicy ScriptLoader::GetReferrerPolicy(nsIScriptElement* aElement) {
+  ReferrerPolicy scriptReferrerPolicy = aElement->GetReferrerPolicy();
+  if (scriptReferrerPolicy != ReferrerPolicy::_empty) {
+    return scriptReferrerPolicy;
+  }
+  return mDocument->GetReferrerPolicy();
+}
+
+void ScriptLoader::CancelAndClearScriptLoadRequests() {
+  // Cancel all requests that have not been executed and remove them.
+
+  if (mParserBlockingRequest) {
+    mParserBlockingRequest->Cancel();
+    mParserBlockingRequest = nullptr;
+  }
+
+  mDeferRequests.CancelRequestsAndClear();
+  mLoadingAsyncRequests.CancelRequestsAndClear();
+  mLoadedAsyncRequests.CancelRequestsAndClear();
+  mNonAsyncExternalScriptInsertedRequests.CancelRequestsAndClear();
+  mXSLTRequests.CancelRequestsAndClear();
+  mOffThreadCompilingRequests.CancelRequestsAndClear();
+
+  if (mModuleLoader) {
+    mModuleLoader->CancelAndClearDynamicImports();
+  }
+
+  for (ModuleLoader* loader : mWebExtModuleLoaders) {
+    loader->CancelAndClearDynamicImports();
+  }
+
+  for (ModuleLoader* loader : mShadowRealmModuleLoaders) {
+    loader->CancelAndClearDynamicImports();
+  }
+
+  for (size_t i = 0; i < mPreloads.Length(); i++) {
+    mPreloads[i].mRequest->Cancel();
+  }
+  mPreloads.Clear();
+}
+
+nsresult ScriptLoader::CompileOffThreadOrProcessRequest(
+    ScriptLoadRequest* aRequest) {
+  NS_ASSERTION(nsContentUtils::IsSafeToRunScript(),
+               "Processing requests when running scripts is unsafe.");
+
+  if (!aRequest->GetScriptLoadContext()->mCompileOrDecodeTask &&
+      !aRequest->GetScriptLoadContext()->CompileStarted()) {
+    bool couldCompile = false;
+    nsresult rv = AttemptOffThreadScriptCompile(aRequest, &couldCompile);
+    if (NS_FAILED(rv)) {
+      HandleLoadError(aRequest, rv);
+      return rv;
+    }
+
+    if (couldCompile) {
+      return NS_OK;
+    }
+  }
+
+  return ProcessRequest(aRequest);
+}
+
+namespace {
+
+class OffThreadCompilationCompleteTask : public Task {
+ public:
+  OffThreadCompilationCompleteTask(ScriptLoadRequest* aRequest,
+                                   ScriptLoader* aLoader)
+      : Task(Kind::MainThreadOnly, EventQueuePriority::Normal),
+        mRequest(aRequest),
+        mLoader(aLoader) {
+    MOZ_ASSERT(NS_IsMainThread());
+  }
+
+  void RecordStartTime() { mStartTime = TimeStamp::Now(); }
+  void RecordStopTime() { mStopTime = TimeStamp::Now(); }
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  bool GetName(nsACString& aName) override {
+    aName.AssignLiteral("dom::OffThreadCompilationCompleteTask");
+    return true;
+  }
+#endif
+
+  TaskResult Run() override {
+    MOZ_ASSERT(NS_IsMainThread());
+
+    RefPtr<ScriptLoadContext> context = mRequest->GetScriptLoadContext();
+
+    if (!context->mCompileOrDecodeTask) {
+      // Request has been cancelled by MaybeCancelOffThreadScript.
+      return TaskResult::Complete;
+    }
+
+    RecordStopTime();
+
+    if (profiler_is_active()) {
+      ProfilerString8View scriptSourceString;
+      if (mRequest->IsTextSource()) {
+        scriptSourceString = "ScriptCompileOffThread";
+      } else {
+        MOZ_ASSERT(mRequest->IsBytecode());
+        scriptSourceString = "BytecodeDecodeOffThread";
+      }
+
+      nsAutoCString profilerLabelString;
+      mRequest->GetScriptLoadContext()->GetProfilerLabel(profilerLabelString);
+      PROFILER_MARKER_TEXT(scriptSourceString, JS,
+                           MarkerTiming::Interval(mStartTime, mStopTime),
+                           profilerLabelString);
+    }
+
+    (void)mLoader->ProcessOffThreadRequest(mRequest);
+
+    mRequest = nullptr;
+    mLoader = nullptr;
+    return TaskResult::Complete;
+  }
+
+ private:
+  // NOTE:
+  // These fields are main-thread only, and this task shouldn't be freed off
+  // main thread.
+  //
+  // This is guaranteed by not having off-thread tasks which depends on this
+  // task, because otherwise the off-thread task's mDependencies can be the
+  // last reference, which results in freeing this task off main thread.
+  //
+  // If such task is added, these fields must be moved to separate storage.
+  RefPtr<ScriptLoadRequest> mRequest;
+  RefPtr<ScriptLoader> mLoader;
+
+  TimeStamp mStartTime;
+  TimeStamp mStopTime;
+};
+
+} /* anonymous namespace */
+
+// TODO: This uses the same heuristics and the same threshold as the
+//       JS::CanCompileOffThread / JS::CanDecodeOffThread APIs, but the
+//       heuristics needs to be updated to reflect the change regarding the
+//       Stencil API, and also the thread management on the consumer side
+//       (bug 1846160).
+static constexpr size_t OffThreadMinimumTextLength = 5 * 1000;
+static constexpr size_t OffThreadMinimumBytecodeLength = 5 * 1000;
+
+nsresult ScriptLoader::AttemptOffThreadScriptCompile(
+    ScriptLoadRequest* aRequest, bool* aCouldCompileOut) {
+  // If speculative parsing is enabled, the request may not be ready to run if
+  // the element is not yet available.
+  MOZ_ASSERT_IF(!SpeculativeOMTParsingEnabled() && !aRequest->IsModuleRequest(),
+                aRequest->IsFinished());
+  MOZ_ASSERT(!aRequest->GetScriptLoadContext()->mWasCompiledOMT);
+  MOZ_ASSERT(aCouldCompileOut && !*aCouldCompileOut);
+
+  // Don't off-thread compile inline scripts.
+  if (aRequest->GetScriptLoadContext()->mIsInline) {
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIGlobalObject> globalObject = GetGlobalForRequest(aRequest);
+  if (!globalObject) {
+    return NS_ERROR_FAILURE;
+  }
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(globalObject)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  JSContext* cx = jsapi.cx();
+  JS::CompileOptions options(cx);
+
+  // Introduction script will actually be computed and set when the script is
+  // collected from offthread
+  JS::Rooted<JSScript*> dummyIntroductionScript(cx);
+  nsresult rv = FillCompileOptionsForRequest(cx, aRequest, &options,
+                                             &dummyIntroductionScript);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  if (aRequest->IsTextSource()) {
+    if (!StaticPrefs::javascript_options_parallel_parsing() ||
+        aRequest->ScriptTextLength() < OffThreadMinimumTextLength) {
+      TRACE_FOR_TEST(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                     "scriptloader_main_thread_compile");
+      return NS_OK;
+    }
+  } else {
+    MOZ_ASSERT(aRequest->IsBytecode());
+
+    JS::TranscodeRange bytecode = aRequest->Bytecode();
+    if (!StaticPrefs::javascript_options_parallel_parsing() ||
+        bytecode.length() < OffThreadMinimumBytecodeLength) {
+      return NS_OK;
+    }
+  }
+
+  RefPtr<CompileOrDecodeTask> compileOrDecodeTask;
+  rv = CreateOffThreadTask(cx, aRequest, options,
+                           getter_AddRefs(compileOrDecodeTask));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  RefPtr<OffThreadCompilationCompleteTask> completeTask =
+      new OffThreadCompilationCompleteTask(aRequest, this);
+
+  completeTask->RecordStartTime();
+
+  aRequest->GetScriptLoadContext()->mCompileOrDecodeTask = compileOrDecodeTask;
+  completeTask->AddDependency(compileOrDecodeTask);
+
+  TaskController::Get()->AddTask(compileOrDecodeTask.forget());
+  TaskController::Get()->AddTask(completeTask.forget());
+
+  aRequest->GetScriptLoadContext()->BlockOnload(mDocument);
+
+  // Once the compilation is finished, the completeTask will be run on
+  // the main thread to call ScriptLoader::ProcessOffThreadRequest for the
+  // request.
+  aRequest->mState = ScriptLoadRequest::State::Compiling;
+
+  // Requests that are not tracked elsewhere are added to a list while they are
+  // being compiled off-thread, so we can cancel the compilation later if
+  // necessary.
+  //
+  // Non-top-level modules not tracked because these are cancelled from their
+  // importing module.
+  if (aRequest->IsTopLevel() && !aRequest->isInList()) {
+    mOffThreadCompilingRequests.AppendElement(aRequest);
+    aRequest->GetScriptLoadContext()->mInCompilingList = true;
+  }
+
+  *aCouldCompileOut = true;
+
+  return NS_OK;
+}
+
+CompileOrDecodeTask::CompileOrDecodeTask()
+    : Task(Kind::OffMainThreadOnly, EventQueuePriority::Normal),
+      mMutex("CompileOrDecodeTask"),
+      mOptions(JS::OwningCompileOptions::ForFrontendContext()) {}
+
+CompileOrDecodeTask::~CompileOrDecodeTask() {
+  if (mFrontendContext) {
+    JS::DestroyFrontendContext(mFrontendContext);
+    mFrontendContext = nullptr;
+  }
+}
+
+nsresult CompileOrDecodeTask::InitFrontendContext() {
+  mFrontendContext = JS::NewFrontendContext();
+  if (!mFrontendContext) {
+    mIsCancelled = true;
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  return NS_OK;
+}
+
+void CompileOrDecodeTask::DidRunTask(const MutexAutoLock& aProofOfLock,
+                                     RefPtr<JS::Stencil>&& aStencil) {
+  if (aStencil) {
+    if (!JS::PrepareForInstantiate(mFrontendContext, *aStencil,
+                                   mInstantiationStorage)) {
+      aStencil = nullptr;
+    }
+  }
+
+  mStencil = std::move(aStencil);
+}
+
+already_AddRefed<JS::Stencil> CompileOrDecodeTask::StealResult(
+    JSContext* aCx, JS::InstantiationStorage* aInstantiationStorage) {
+  JS::FrontendContext* fc = mFrontendContext;
+  mFrontendContext = nullptr;
+  auto destroyFrontendContext =
+      mozilla::MakeScopeExit([&]() { JS::DestroyFrontendContext(fc); });
+
+  MOZ_ASSERT(fc);
+
+  if (JS::HadFrontendErrors(fc)) {
+    (void)JS::ConvertFrontendErrorsToRuntimeErrors(aCx, fc, mOptions);
+    return nullptr;
+  }
+
+  if (!mStencil && JS::IsTranscodeFailureResult(mResult)) {
+    // Decode failure with bad content isn't reported as error.
+    JS_ReportErrorASCII(aCx, "failed to decode cache");
+    return nullptr;
+  }
+
+  // Report warnings.
+  if (!JS::ConvertFrontendErrorsToRuntimeErrors(aCx, fc, mOptions)) {
+    return nullptr;
+  }
+
+  MOZ_ASSERT(mStencil,
+             "If this task is cancelled, StealResult shouldn't be called");
+
+  // This task is started and finished successfully.
+  *aInstantiationStorage = std::move(mInstantiationStorage);
+
+  return mStencil.forget();
+}
+
+void CompileOrDecodeTask::Cancel() {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  MutexAutoLock lock(mMutex);
+
+  mIsCancelled = true;
+}
+
+enum class CompilationTarget { Script, Module };
+
+template <CompilationTarget target>
+class ScriptOrModuleCompileTask final : public CompileOrDecodeTask {
+ public:
+  explicit ScriptOrModuleCompileTask(
+      ScriptLoader::MaybeSourceText&& aMaybeSource)
+      : CompileOrDecodeTask(), mMaybeSource(std::move(aMaybeSource)) {}
+
+  nsresult Init(JS::CompileOptions& aOptions) {
+    nsresult rv = InitFrontendContext();
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    if (!mOptions.copy(mFrontendContext, aOptions)) {
+      mIsCancelled = true;
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    return NS_OK;
+  }
+
+  TaskResult Run() override {
+    MutexAutoLock lock(mMutex);
+
+    if (IsCancelled(lock)) {
+      return TaskResult::Complete;
+    }
+
+    RefPtr<JS::Stencil> stencil = Compile();
+
+    DidRunTask(lock, std::move(stencil));
+    return TaskResult::Complete;
+  }
+
+ private:
+  already_AddRefed<JS::Stencil> Compile() {
+    size_t stackSize = TaskController::GetThreadStackSize();
+    JS::SetNativeStackQuota(mFrontendContext,
+                            JS::ThreadStackQuotaForSize(stackSize));
+
+    JS::CompilationStorage compileStorage;
+    auto compile = [&](auto& source) {
+      if constexpr (target == CompilationTarget::Script) {
+        return JS::CompileGlobalScriptToStencil(mFrontendContext, mOptions,
+                                                source, compileStorage);
+      }
+      return JS::CompileModuleScriptToStencil(mFrontendContext, mOptions,
+                                              source, compileStorage);
+    };
+    return mMaybeSource.mapNonEmpty(compile);
+  }
+
+ public:
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  bool GetName(nsACString& aName) override {
+    if constexpr (target == CompilationTarget::Script) {
+      aName.AssignLiteral("ScriptCompileTask");
+    } else {
+      aName.AssignLiteral("ModuleCompileTask");
+    }
+    return true;
+  }
+#endif
+
+ private:
+  ScriptLoader::MaybeSourceText mMaybeSource;
+};
+
+using ScriptCompileTask =
+    class ScriptOrModuleCompileTask<CompilationTarget::Script>;
+using ModuleCompileTask =
+    class ScriptOrModuleCompileTask<CompilationTarget::Module>;
+
+class ScriptDecodeTask final : public CompileOrDecodeTask {
+ public:
+  explicit ScriptDecodeTask(const JS::TranscodeRange& aRange)
+      : mRange(aRange) {}
+
+  nsresult Init(JS::DecodeOptions& aOptions) {
+    nsresult rv = InitFrontendContext();
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    if (!mDecodeOptions.copy(mFrontendContext, aOptions)) {
+      mIsCancelled = true;
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    return NS_OK;
+  }
+
+  TaskResult Run() override {
+    MutexAutoLock lock(mMutex);
+
+    if (IsCancelled(lock)) {
+      return TaskResult::Complete;
+    }
+
+    RefPtr<JS::Stencil> stencil = Decode();
+
+    JS::OwningCompileOptions compileOptions(
+        (JS::OwningCompileOptions::ForFrontendContext()));
+    mOptions.steal(std::move(mDecodeOptions));
+
+    DidRunTask(lock, std::move(stencil));
+    return TaskResult::Complete;
+  }
+
+ private:
+  already_AddRefed<JS::Stencil> Decode() {
+    // NOTE: JS::DecodeStencil doesn't need the stack quota.
+
+    JS::CompilationStorage compileStorage;
+    RefPtr<JS::Stencil> stencil;
+    mResult = JS::DecodeStencil(mFrontendContext, mDecodeOptions, mRange,
+                                getter_AddRefs(stencil));
+    return stencil.forget();
+  }
+
+ public:
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  bool GetName(nsACString& aName) override {
+    aName.AssignLiteral("ScriptDecodeTask");
+    return true;
+  }
+#endif
+
+ private:
+  JS::OwningDecodeOptions mDecodeOptions;
+
+  JS::TranscodeRange mRange;
+};
+
+nsresult ScriptLoader::CreateOffThreadTask(
+    JSContext* aCx, ScriptLoadRequest* aRequest, JS::CompileOptions& aOptions,
+    CompileOrDecodeTask** aCompileOrDecodeTask) {
+  if (aRequest->IsBytecode()) {
+    JS::TranscodeRange bytecode = aRequest->Bytecode();
+    JS::DecodeOptions decodeOptions(aOptions);
+    RefPtr<ScriptDecodeTask> decodeTask = new ScriptDecodeTask(bytecode);
+    nsresult rv = decodeTask->Init(decodeOptions);
+    NS_ENSURE_SUCCESS(rv, rv);
+    decodeTask.forget(aCompileOrDecodeTask);
+    return NS_OK;
+  }
+
+  MaybeSourceText maybeSource;
+  nsresult rv = aRequest->GetScriptSource(aCx, &maybeSource,
+                                          aRequest->mLoadContext.get());
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (ShouldApplyDelazifyStrategy(aRequest)) {
+    ApplyDelazifyStrategy(&aOptions);
+    mTotalFullParseSize +=
+        aRequest->ScriptTextLength() > 0
+            ? static_cast<uint32_t>(aRequest->ScriptTextLength())
+            : 0;
+
+    LOG(
+        ("ScriptLoadRequest (%p): non-on-demand-only (omt) Parsing Enabled "
+         "for url=%s mTotalFullParseSize=%u",
+         aRequest, aRequest->mURI->GetSpecOrDefault().get(),
+         mTotalFullParseSize));
+  }
+
+  if (aRequest->IsModuleRequest()) {
+    RefPtr<ModuleCompileTask> compileTask =
+        new ModuleCompileTask(std::move(maybeSource));
+    rv = compileTask->Init(aOptions);
+    NS_ENSURE_SUCCESS(rv, rv);
+    compileTask.forget(aCompileOrDecodeTask);
+    return NS_OK;
+  }
+
+  if (StaticPrefs::dom_expose_test_interfaces()) {
+    switch (aOptions.eagerDelazificationStrategy()) {
+      case JS::DelazificationOption::OnDemandOnly:
+        TRACE_FOR_TEST(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                       "delazification_on_demand_only");
+        break;
+      case JS::DelazificationOption::CheckConcurrentWithOnDemand:
+      case JS::DelazificationOption::ConcurrentDepthFirst:
+        TRACE_FOR_TEST(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                       "delazification_concurrent_depth_first");
+        break;
+      case JS::DelazificationOption::ConcurrentLargeFirst:
+        TRACE_FOR_TEST(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                       "delazification_concurrent_large_first");
+        break;
+      case JS::DelazificationOption::ParseEverythingEagerly:
+        TRACE_FOR_TEST(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                       "delazification_parse_everything_eagerly");
+        break;
+    }
+  }
+
+  RefPtr<ScriptCompileTask> compileTask =
+      new ScriptCompileTask(std::move(maybeSource));
+  rv = compileTask->Init(aOptions);
+  NS_ENSURE_SUCCESS(rv, rv);
+  compileTask.forget(aCompileOrDecodeTask);
+  return NS_OK;
+}
+
+nsresult ScriptLoader::ProcessOffThreadRequest(ScriptLoadRequest* aRequest) {
+  MOZ_ASSERT(aRequest->IsCompiling());
+  MOZ_ASSERT(!aRequest->GetScriptLoadContext()->mWasCompiledOMT);
+
+  if (aRequest->IsCanceled()) {
+    return NS_OK;
+  }
+
+  aRequest->GetScriptLoadContext()->mWasCompiledOMT = true;
+
+  if (aRequest->GetScriptLoadContext()->mInCompilingList) {
+    mOffThreadCompilingRequests.Remove(aRequest);
+    aRequest->GetScriptLoadContext()->mInCompilingList = false;
+  }
+
+  if (aRequest->IsModuleRequest()) {
+    MOZ_ASSERT(aRequest->GetScriptLoadContext()->mCompileOrDecodeTask);
+    ModuleLoadRequest* request = aRequest->AsModuleRequest();
+    return request->OnFetchComplete(NS_OK);
+  }
+
+  // Element may not be ready yet if speculatively compiling, so process the
+  // request in ProcessPendingRequests when it is available.
+  MOZ_ASSERT_IF(!SpeculativeOMTParsingEnabled(),
+                aRequest->GetScriptLoadContext()->GetScriptElement());
+  if (!aRequest->GetScriptLoadContext()->GetScriptElement()) {
+    // Unblock onload here in case this request never gets executed.
+    aRequest->GetScriptLoadContext()->MaybeUnblockOnload();
+    return NS_OK;
+  }
+
+  aRequest->SetReady();
+
+  if (aRequest == mParserBlockingRequest) {
+    if (!ReadyToExecuteParserBlockingScripts()) {
+      // If not ready to execute scripts, schedule an async call to
+      // ProcessPendingRequests to handle it.
+      ProcessPendingRequestsAsync();
+      return NS_OK;
+    }
+
+    // Same logic as in top of ProcessPendingRequests.
+    mParserBlockingRequest = nullptr;
+    UnblockParser(aRequest);
+    ProcessRequest(aRequest);
+    ContinueParserAsync(aRequest);
+    return NS_OK;
+  }
+
+  // Async scripts and blocking scripts can be executed right away.
+  if ((aRequest->GetScriptLoadContext()->IsAsyncScript() ||
+       aRequest->GetScriptLoadContext()->IsBlockingScript()) &&
+      !aRequest->isInList()) {
+    return ProcessRequest(aRequest);
+  }
+
+  // Process other scripts in the proper order.
+  ProcessPendingRequests();
+  return NS_OK;
+}
+
+nsresult ScriptLoader::ProcessRequest(ScriptLoadRequest* aRequest) {
+  LOG(("ScriptLoadRequest (%p): Process request", aRequest));
+
+  NS_ASSERTION(nsContentUtils::IsSafeToRunScript(),
+               "Processing requests when running scripts is unsafe.");
+  NS_ASSERTION(aRequest->IsFinished(),
+               "Processing a request that is not ready to run.");
+
+  NS_ENSURE_ARG(aRequest);
+
+  auto unblockOnload = MakeScopeExit(
+      [&] { aRequest->GetScriptLoadContext()->MaybeUnblockOnload(); });
+
+  if (aRequest->IsModuleRequest()) {
+    ModuleLoadRequest* request = aRequest->AsModuleRequest();
+    if (request->IsDynamicImport()) {
+      request->ProcessDynamicImport();
+      return NS_OK;
+    }
+
+    if (request->mModuleScript) {
+      if (!request->InstantiateModuleGraph()) {
+        request->mModuleScript = nullptr;
+      }
+    }
+
+    if (!request->mModuleScript) {
+      // There was an error fetching a module script.  Nothing to do here.
+      LOG(("ScriptLoadRequest (%p):   Error loading request, firing error",
+           aRequest));
+      FireScriptAvailable(NS_ERROR_FAILURE, aRequest);
+      return NS_OK;
+    }
+  }
+
+  nsCOMPtr<nsINode> scriptElem =
+      do_QueryInterface(aRequest->GetScriptLoadContext()->GetScriptElement());
+
+  nsCOMPtr<Document> doc;
+  if (!aRequest->GetScriptLoadContext()->mIsInline ||
+      aRequest->IsModuleRequest()) {
+    doc = scriptElem->OwnerDoc();
+  }
+
+  nsCOMPtr<nsIScriptElement> oldParserInsertedScript;
+  uint32_t parserCreated = aRequest->GetScriptLoadContext()->GetParserCreated();
+  if (parserCreated) {
+    oldParserInsertedScript = mCurrentParserInsertedScript;
+    mCurrentParserInsertedScript =
+        aRequest->GetScriptLoadContext()->GetScriptElement();
+  }
+
+  aRequest->GetScriptLoadContext()->GetScriptElement()->BeginEvaluating();
+
+  FireScriptAvailable(NS_OK, aRequest);
+
+  // The window may have gone away by this point, in which case there's no point
+  // in trying to run the script.
+
+  {
+    // Try to perform a microtask checkpoint
+    nsAutoMicroTask mt;
+  }
+
+  nsPIDOMWindowInner* pwin = mDocument->GetInnerWindow();
+  bool runScript = !!pwin;
+  if (runScript) {
+    nsContentUtils::DispatchTrustedEvent(
+        scriptElem->OwnerDoc(), scriptElem, u"beforescriptexecute"_ns,
+        CanBubble::eYes, Cancelable::eYes, &runScript);
+  }
+
+  // Inner window could have gone away after firing beforescriptexecute
+  pwin = mDocument->GetInnerWindow();
+  if (!pwin) {
+    runScript = false;
+  }
+
+  nsresult rv = NS_OK;
+  if (runScript) {
+    if (doc) {
+      doc->IncrementIgnoreDestructiveWritesCounter();
+    }
+    rv = EvaluateScriptElement(aRequest);
+    if (doc) {
+      doc->DecrementIgnoreDestructiveWritesCounter();
+    }
+
+    nsContentUtils::DispatchTrustedEvent(scriptElem->OwnerDoc(), scriptElem,
+                                         u"afterscriptexecute"_ns,
+                                         CanBubble::eYes, Cancelable::eNo);
+  }
+
+  FireScriptEvaluated(rv, aRequest);
+
+  aRequest->GetScriptLoadContext()->GetScriptElement()->EndEvaluating();
+
+  if (parserCreated) {
+    mCurrentParserInsertedScript = oldParserInsertedScript;
+  }
+
+  if (aRequest->GetScriptLoadContext()->mCompileOrDecodeTask) {
+    // The request was parsed off-main-thread, but the result of the off
+    // thread parse was not actually needed to process the request
+    // (disappearing window, some other error, ...). Finish the
+    // request to avoid leaks.
+    MOZ_ASSERT(!aRequest->IsModuleRequest());
+    aRequest->GetScriptLoadContext()->MaybeCancelOffThreadScript();
+  }
+
+  // Free any source data, but keep the bytecode content as we might have to
+  // save it later.
+  aRequest->ClearScriptSource();
+  if (aRequest->IsBytecode()) {
+    // We received bytecode as input, thus we were decoding, and we will not be
+    // encoding the bytecode once more. We can safely clear the content of this
+    // buffer.
+    aRequest->DropBytecode();
+  }
+
+  return rv;
+}
+
+void ScriptLoader::FireScriptAvailable(nsresult aResult,
+                                       ScriptLoadRequest* aRequest) {
+  for (int32_t i = 0; i < mObservers.Count(); i++) {
+    nsCOMPtr<nsIScriptLoaderObserver> obs = mObservers[i];
+    obs->ScriptAvailable(
+        aResult, aRequest->GetScriptLoadContext()->GetScriptElement(),
+        aRequest->GetScriptLoadContext()->mIsInline, aRequest->mURI,
+        aRequest->GetScriptLoadContext()->mLineNo);
+  }
+
+  bool isInlineClassicScript = aRequest->GetScriptLoadContext()->mIsInline &&
+                               !aRequest->IsModuleRequest();
+  RefPtr<nsIScriptElement> scriptElement =
+      aRequest->GetScriptLoadContext()->GetScriptElement();
+  scriptElement->ScriptAvailable(aResult, scriptElement, isInlineClassicScript,
+                                 aRequest->mURI,
+                                 aRequest->GetScriptLoadContext()->mLineNo);
+}
+
+// TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230)
+MOZ_CAN_RUN_SCRIPT_BOUNDARY void ScriptLoader::FireScriptEvaluated(
+    nsresult aResult, ScriptLoadRequest* aRequest) {
+  for (int32_t i = 0; i < mObservers.Count(); i++) {
+    nsCOMPtr<nsIScriptLoaderObserver> obs = mObservers[i];
+    RefPtr<nsIScriptElement> scriptElement =
+        aRequest->GetScriptLoadContext()->GetScriptElement();
+    obs->ScriptEvaluated(aResult, scriptElement,
+                         aRequest->GetScriptLoadContext()->mIsInline);
+  }
+
+  RefPtr<nsIScriptElement> scriptElement =
+      aRequest->GetScriptLoadContext()->GetScriptElement();
+  scriptElement->ScriptEvaluated(aResult, scriptElement,
+                                 aRequest->GetScriptLoadContext()->mIsInline);
+}
+
+already_AddRefed<nsIGlobalObject> ScriptLoader::GetGlobalForRequest(
+    ScriptLoadRequest* aRequest) {
+  if (aRequest->IsModuleRequest()) {
+    ModuleLoader* loader =
+        ModuleLoader::From(aRequest->AsModuleRequest()->mLoader);
+    nsCOMPtr<nsIGlobalObject> global = loader->GetGlobalObject();
+    return global.forget();
+  }
+
+  return GetScriptGlobalObject();
+}
+
+already_AddRefed<nsIScriptGlobalObject> ScriptLoader::GetScriptGlobalObject() {
+  if (!mDocument) {
+    return nullptr;
+  }
+
+  nsPIDOMWindowInner* pwin = mDocument->GetInnerWindow();
+  if (!pwin) {
+    return nullptr;
+  }
+
+  nsCOMPtr<nsIScriptGlobalObject> globalObject = do_QueryInterface(pwin);
+  NS_ASSERTION(globalObject, "windows must be global objects");
+
+  // and make sure we are setup for this type of script.
+  nsresult rv = globalObject->EnsureScriptEnvironment();
+  if (NS_FAILED(rv)) {
+    return nullptr;
+  }
+
+  return globalObject.forget();
+}
+
+nsresult ScriptLoader::FillCompileOptionsForRequest(
+    JSContext* aCx, ScriptLoadRequest* aRequest, JS::CompileOptions* aOptions,
+    JS::MutableHandle<JSScript*> aIntroductionScript) {
+  // It's very important to use aRequest->mURI, not the final URI of the channel
+  // aRequest ended up getting script data from, as the script filename.
+  nsresult rv = aRequest->mURI->GetSpec(aRequest->mURL);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  if (mDocument) {
+    mDocument->NoteScriptTrackingStatus(
+        aRequest->mURL, aRequest->GetScriptLoadContext()->IsTracking());
+  }
+
+  const char* introductionType;
+  if (aRequest->IsModuleRequest() &&
+      !aRequest->AsModuleRequest()->IsTopLevel()) {
+    introductionType = "importedModule";
+  } else if (!aRequest->GetScriptLoadContext()->mIsInline) {
+    introductionType = "srcScript";
+  } else if (aRequest->GetScriptLoadContext()->GetParserCreated() ==
+             FROM_PARSER_NETWORK) {
+    introductionType = "inlineScript";
+  } else {
+    introductionType = "injectedScript";
+  }
+  aOptions->setIntroductionInfoToCaller(aCx, introductionType,
+                                        aIntroductionScript);
+  aOptions->setFileAndLine(aRequest->mURL.get(),
+                           aRequest->GetScriptLoadContext()->mLineNo);
+  // The column is only relevant for inline scripts in order for SpiderMonkey to
+  // properly compute offsets relatively to the script position within the HTML
+  // file. injectedScript are not concerned and are always considered to start
+  // at column 0.
+  if (aRequest->GetScriptLoadContext()->mIsInline &&
+      aRequest->GetScriptLoadContext()->GetParserCreated() ==
+          FROM_PARSER_NETWORK) {
+    aOptions->setColumn(aRequest->GetScriptLoadContext()->mColumnNo);
+  }
+  aOptions->setIsRunOnce(true);
+  aOptions->setNoScriptRval(true);
+  if (aRequest->mSourceMapURL) {
+    aOptions->setSourceMapURL(aRequest->mSourceMapURL->get());
+  }
+  if (aRequest->mOriginPrincipal) {
+    nsCOMPtr<nsIGlobalObject> globalObject = GetGlobalForRequest(aRequest);
+    nsIPrincipal* scriptPrin = globalObject->PrincipalOrNull();
+    MOZ_ASSERT(scriptPrin);
+    bool subsumes = scriptPrin->Subsumes(aRequest->mOriginPrincipal);
+    aOptions->setMutedErrors(!subsumes);
+  }
+
+  if (aRequest->IsModuleRequest()) {
+    aOptions->setHideScriptFromDebugger(true);
+  }
+
+  aOptions->setDeferDebugMetadata(true);
+
+  aOptions->borrowBuffer = true;
+
+  return NS_OK;
+}
+
+/* static */
+bool ScriptLoader::ShouldCacheBytecode(ScriptLoadRequest* aRequest) {
+  using mozilla::TimeDuration;
+  using mozilla::TimeStamp;
+
+  // We need the nsICacheInfoChannel to exist to be able to open the alternate
+  // data output stream. This pointer would only be non-null if the bytecode was
+  // activated at the time the channel got created in StartLoad.
+  if (!aRequest->mCacheInfo) {
+    LOG(("ScriptLoadRequest (%p): Cannot cache anything (cacheInfo = %p)",
+         aRequest, aRequest->mCacheInfo.get()));
+    return false;
+  }
+
+  // Look at the preference to know which strategy (parameters) should be used
+  // when the bytecode cache is enabled.
+  int32_t strategy = StaticPrefs::dom_script_loader_bytecode_cache_strategy();
+
+  // List of parameters used by the strategies.
+  bool hasSourceLengthMin = false;
+  bool hasFetchCountMin = false;
+  size_t sourceLengthMin = 100;
+  uint32_t fetchCountMin = 4;
+
+  LOG(("ScriptLoadRequest (%p): Bytecode-cache: strategy = %d.", aRequest,
+       strategy));
+  switch (strategy) {
+    case -2: {
+      // Reader mode, keep requesting alternate data but no longer save it.
+      LOG(("ScriptLoadRequest (%p): Bytecode-cache: Encoding disabled.",
+           aRequest));
+      return false;
+    }
+    case -1: {
+      // Eager mode, skip heuristics!
+      hasSourceLengthMin = false;
+      hasFetchCountMin = false;
+      break;
+    }
+    default:
+    case 0: {
+      hasSourceLengthMin = true;
+      hasFetchCountMin = true;
+      sourceLengthMin = 1024;
+      // If we were to optimize only for speed, without considering the impact
+      // on memory, we should set this threshold to 2. (Bug 900784 comment 120)
+      fetchCountMin = 4;
+      break;
+    }
+  }
+
+  // If the script is too small/large, do not attempt at creating a bytecode
+  // cache for this script, as the overhead of parsing it might not be worth the
+  // effort.
+  if (hasSourceLengthMin) {
+    size_t sourceLength;
+    size_t minLength;
+    MOZ_ASSERT(aRequest->IsTextSource());
+    sourceLength = aRequest->ReceivedScriptTextLength();
+    minLength = sourceLengthMin;
+    if (sourceLength < minLength) {
+      LOG(("ScriptLoadRequest (%p): Bytecode-cache: Script is too small.",
+           aRequest));
+      return false;
+    }
+  }
+
+  // Check that we loaded the cache entry a few times before attempting any
+  // bytecode-cache optimization, such that we do not waste time on entry which
+  // are going to be dropped soon.
+  if (hasFetchCountMin) {
+    uint32_t fetchCount = 0;
+    if (NS_FAILED(aRequest->mCacheInfo->GetCacheTokenFetchCount(&fetchCount))) {
+      LOG(("ScriptLoadRequest (%p): Bytecode-cache: Cannot get fetchCount.",
+           aRequest));
+      return false;
+    }
+    LOG(("ScriptLoadRequest (%p): Bytecode-cache: fetchCount = %d.", aRequest,
+         fetchCount));
+    if (fetchCount < fetchCountMin) {
+      return false;
+    }
+  }
+
+  LOG(("ScriptLoadRequest (%p): Bytecode-cache: Trigger encoding.", aRequest));
+  return true;
+}
+
+class MOZ_RAII AutoSetProcessingScriptTag {
+  nsCOMPtr<nsIScriptContext> mContext;
+  bool mOldTag;
+
+ public:
+  explicit AutoSetProcessingScriptTag(nsIScriptContext* aContext)
+      : mContext(aContext), mOldTag(mContext->GetProcessingScriptTag()) {
+    mContext->SetProcessingScriptTag(true);
+  }
+
+  ~AutoSetProcessingScriptTag() { mContext->SetProcessingScriptTag(mOldTag); }
+};
+
+static nsresult ExecuteCompiledScript(JSContext* aCx, JSExecutionContext& aExec,
+                                      ClassicScript* aLoaderScript) {
+  JS::Rooted<JSScript*> script(aCx, aExec.GetScript());
+  if (!script) {
+    // Compilation succeeds without producing a script if scripting is
+    // disabled for the global.
+    return NS_OK;
+  }
+
+  if (JS::GetScriptPrivate(script).isUndefined()) {
+    aLoaderScript->AssociateWithScript(script);
+  }
+
+  return aExec.ExecScript();
+}
+
+nsresult ScriptLoader::EvaluateScriptElement(ScriptLoadRequest* aRequest) {
+  using namespace mozilla::Telemetry;
+  MOZ_ASSERT(aRequest->IsFinished());
+
+  // We need a document to evaluate scripts.
+  if (!mDocument) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<nsIContent> scriptContent(
+      do_QueryInterface(aRequest->GetScriptLoadContext()->GetScriptElement()));
+  MOZ_ASSERT(scriptContent);
+  Document* ownerDoc = scriptContent->OwnerDoc();
+  if (ownerDoc != mDocument) {
+    // Willful violation of HTML5 as of 2010-12-01
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<nsIGlobalObject> globalObject;
+  nsCOMPtr<nsIScriptContext> context;
+  if (!IsWebExtensionRequest(aRequest)) {
+    // Otherwise we have to ensure that there is a nsIScriptContext.
+    nsCOMPtr<nsIScriptGlobalObject> scriptGlobal = GetScriptGlobalObject();
+    if (!scriptGlobal) {
+      return NS_ERROR_FAILURE;
+    }
+
+    MOZ_ASSERT_IF(
+        aRequest->IsModuleRequest(),
+        aRequest->AsModuleRequest()->GetGlobalObject() == scriptGlobal);
+
+    // Make sure context is a strong reference since we access it after
+    // we've executed a script, which may cause all other references to
+    // the context to go away.
+    context = scriptGlobal->GetScriptContext();
+    if (!context) {
+      return NS_ERROR_FAILURE;
+    }
+
+    globalObject = scriptGlobal;
+  }
+
+  // Update our current script.
+  // This must be destroyed after destroying nsAutoMicroTask, see:
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=1620505#c4
+  nsIScriptElement* currentScript =
+      aRequest->IsModuleRequest()
+          ? nullptr
+          : aRequest->GetScriptLoadContext()->GetScriptElement();
+  AutoCurrentScriptUpdater scriptUpdater(this, currentScript);
+
+  Maybe<AutoSetProcessingScriptTag> setProcessingScriptTag;
+  if (context) {
+    setProcessingScriptTag.emplace(context);
+  }
+
+  // https://wicg.github.io/import-maps/#integration-script-type
+  // Switch on the script's type for scriptElement:
+  // "importmap"
+  //    Assert: Never reached.
+  MOZ_ASSERT(!aRequest->IsImportMapRequest());
+
+  if (aRequest->IsModuleRequest()) {
+    return aRequest->AsModuleRequest()->EvaluateModule();
+  }
+
+  return EvaluateScript(globalObject, aRequest);
+}
+
+nsresult ScriptLoader::CompileOrDecodeClassicScript(
+    JSContext* aCx, JSExecutionContext& aExec, ScriptLoadRequest* aRequest) {
+  nsAutoCString profilerLabelString;
+  aRequest->GetScriptLoadContext()->GetProfilerLabel(profilerLabelString);
+
+  nsresult rv;
+  if (aRequest->IsBytecode()) {
+    if (aRequest->GetScriptLoadContext()->mCompileOrDecodeTask) {
+      LOG(("ScriptLoadRequest (%p): Decode Bytecode & instantiate and Execute",
+           aRequest));
+      rv = aExec.JoinOffThread(aRequest->GetScriptLoadContext());
+    } else {
+      LOG(("ScriptLoadRequest (%p): Decode Bytecode and Execute", aRequest));
+      AUTO_PROFILER_MARKER_TEXT("BytecodeDecodeMainThread", JS,
+                                MarkerInnerWindowIdFromJSContext(aCx),
+                                profilerLabelString);
+
+      rv = aExec.Decode(aRequest->Bytecode());
+    }
+
+    // We do not expect to be saving anything when we already have some
+    // bytecode.
+    MOZ_ASSERT(!aRequest->mCacheInfo);
+    return rv;
+  }
+
+  MOZ_ASSERT(aRequest->IsSource());
+  bool encodeBytecode = ShouldCacheBytecode(aRequest);
+  aExec.SetEncodeBytecode(encodeBytecode);
+
+  if (aRequest->GetScriptLoadContext()->mCompileOrDecodeTask) {
+    // Off-main-thread parsing.
+    LOG(
+        ("ScriptLoadRequest (%p): instantiate off-thread result and "
+         "Execute",
+         aRequest));
+    MOZ_ASSERT(aRequest->IsTextSource());
+    rv = aExec.JoinOffThread(aRequest->GetScriptLoadContext());
+  } else {
+    // Main thread parsing (inline and small scripts)
+    LOG(("ScriptLoadRequest (%p): Compile And Exec", aRequest));
+    MOZ_ASSERT(aRequest->IsTextSource());
+    MaybeSourceText maybeSource;
+    rv = aRequest->GetScriptSource(aCx, &maybeSource,
+                                   aRequest->mLoadContext.get());
+    if (NS_SUCCEEDED(rv)) {
+      AUTO_PROFILER_MARKER_TEXT("ScriptCompileMainThread", JS,
+                                MarkerInnerWindowIdFromJSContext(aCx),
+                                profilerLabelString);
+
+      auto compile = [&](auto& source) { return aExec.Compile(source); };
+
+      MOZ_ASSERT(!maybeSource.empty());
+      TimeStamp startTime = TimeStamp::Now();
+      rv = maybeSource.mapNonEmpty(compile);
+      mMainThreadParseTime += TimeStamp::Now() - startTime;
+    }
+  }
+  return rv;
+}
+
+/* static */
+nsCString& ScriptLoader::BytecodeMimeTypeFor(ScriptLoadRequest* aRequest) {
+  if (aRequest->IsModuleRequest()) {
+    return nsContentUtils::JSModuleBytecodeMimeType();
+  }
+  return nsContentUtils::JSScriptBytecodeMimeType();
+}
+
+void ScriptLoader::MaybePrepareForBytecodeEncodingBeforeExecute(
+    ScriptLoadRequest* aRequest, JS::Handle<JSScript*> aScript) {
+  if (!ShouldCacheBytecode(aRequest)) {
+    return;
+  }
+
+  aRequest->MarkForBytecodeEncoding(aScript);
+}
+
+nsresult ScriptLoader::MaybePrepareForBytecodeEncodingAfterExecute(
+    ScriptLoadRequest* aRequest, nsresult aRv) {
+  if (aRequest->IsMarkedForBytecodeEncoding()) {
+    TRACE_FOR_TEST(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                   "scriptloader_encode");
+    // Check that the TranscodeBuffer which is going to receive the encoded
+    // bytecode only contains the SRI, and nothing more.
+    //
+    // NOTE: This assertion will fail once we start encoding more data after the
+    //       first encode.
+    MOZ_ASSERT(aRequest->GetSRILength() == aRequest->SRIAndBytecode().length());
+    RegisterForBytecodeEncoding(aRequest);
+    MOZ_ASSERT(IsAlreadyHandledForBytecodeEncodingPreparation(aRequest));
+
+    return aRv;
+  }
+
+  LOG(("ScriptLoadRequest (%p): Bytecode-cache: disabled (rv = %X)", aRequest,
+       unsigned(aRv)));
+  TRACE_FOR_TEST_NONE(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                      "scriptloader_no_encode");
+  aRequest->mCacheInfo = nullptr;
+  MOZ_ASSERT(IsAlreadyHandledForBytecodeEncodingPreparation(aRequest));
+
+  return aRv;
+}
+
+bool ScriptLoader::IsAlreadyHandledForBytecodeEncodingPreparation(
+    ScriptLoadRequest* aRequest) {
+  return aRequest->isInList() || !aRequest->mCacheInfo;
+}
+
+void ScriptLoader::MaybePrepareModuleForBytecodeEncodingBeforeExecute(
+    JSContext* aCx, ModuleLoadRequest* aRequest) {
+  {
+    ModuleScript* moduleScript = aRequest->mModuleScript;
+    JS::Rooted<JSObject*> module(aCx, moduleScript->ModuleRecord());
+
+    if (aRequest->IsMarkedForBytecodeEncoding()) {
+      // This module is imported multiple times, and already marked.
+      return;
+    }
+
+    if (ShouldCacheBytecode(aRequest)) {
+      aRequest->MarkModuleForBytecodeEncoding();
+    }
+  }
+
+  for (ModuleLoadRequest* childRequest : aRequest->mImports) {
+    MaybePrepareModuleForBytecodeEncodingBeforeExecute(aCx, childRequest);
+  }
+}
+
+nsresult ScriptLoader::MaybePrepareModuleForBytecodeEncodingAfterExecute(
+    ModuleLoadRequest* aRequest, nsresult aRv) {
+  if (IsAlreadyHandledForBytecodeEncodingPreparation(aRequest)) {
+    // This module is imported multiple times and already handled.
+    return aRv;
+  }
+
+  aRv = MaybePrepareForBytecodeEncodingAfterExecute(aRequest, aRv);
+
+  for (ModuleLoadRequest* childRequest : aRequest->mImports) {
+    aRv = MaybePrepareModuleForBytecodeEncodingAfterExecute(childRequest, aRv);
+  }
+
+  return aRv;
+}
+
+nsresult ScriptLoader::EvaluateScript(nsIGlobalObject* aGlobalObject,
+                                      ScriptLoadRequest* aRequest) {
+  nsAutoMicroTask mt;
+  AutoEntryScript aes(aGlobalObject, "EvaluateScript", true);
+  JSContext* cx = aes.cx();
+
+  nsAutoCString profilerLabelString;
+  aRequest->GetScriptLoadContext()->GetProfilerLabel(profilerLabelString);
+
+  // Create a ClassicScript object and associate it with the JSScript.
+  MOZ_ASSERT(aRequest->mLoadedScript->IsClassicScript());
+  MOZ_ASSERT(aRequest->mLoadedScript->GetFetchOptions() ==
+             aRequest->mFetchOptions);
+  MOZ_ASSERT(aRequest->mLoadedScript->GetURI() == aRequest->mURI);
+  aRequest->mLoadedScript->SetBaseURL(aRequest->mBaseURL);
+  RefPtr<ClassicScript> classicScript =
+      aRequest->mLoadedScript->AsClassicScript();
+  JS::Rooted<JS::Value> classicScriptValue(cx, JS::PrivateValue(classicScript));
+
+  JS::CompileOptions options(cx);
+  JS::Rooted<JSScript*> introductionScript(cx);
+  nsresult rv =
+      FillCompileOptionsForRequest(cx, aRequest, &options, &introductionScript);
+
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // Apply the delazify strategy if the script is small.
+  if (aRequest->IsTextSource() &&
+      aRequest->ScriptTextLength() < OffThreadMinimumTextLength &&
+      ShouldApplyDelazifyStrategy(aRequest)) {
+    ApplyDelazifyStrategy(&options);
+    mTotalFullParseSize +=
+        aRequest->ScriptTextLength() > 0
+            ? static_cast<uint32_t>(aRequest->ScriptTextLength())
+            : 0;
+
+    LOG(
+        ("ScriptLoadRequest (%p): non-on-demand-only (non-omt) Parsing Enabled "
+         "for url=%s mTotalFullParseSize=%u",
+         aRequest, aRequest->mURI->GetSpecOrDefault().get(),
+         mTotalFullParseSize));
+  }
+
+  TRACE_FOR_TEST(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                 "scriptloader_execute");
+  JS::Rooted<JSObject*> global(cx, aGlobalObject->GetGlobalJSObject());
+  JSExecutionContext exec(cx, global, options, classicScriptValue,
+                          introductionScript);
+
+  rv = CompileOrDecodeClassicScript(cx, exec, aRequest);
+
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // TODO (yulia): rewrite this section. rv can be a failing pattern other than
+  // NS_OK which will pass the NS_FAILED check above. If we call exec.GetScript
+  // in that case, it will crash.
+  if (rv == NS_OK) {
+    JS::Rooted<JSScript*> script(cx, exec.GetScript());
+    MaybePrepareForBytecodeEncodingBeforeExecute(aRequest, script);
+
+    {
+      LOG(("ScriptLoadRequest (%p): Evaluate Script", aRequest));
+      AUTO_PROFILER_MARKER_TEXT("ScriptExecution", JS,
+                                MarkerInnerWindowIdFromJSContext(cx),
+                                profilerLabelString);
+
+      rv = ExecuteCompiledScript(cx, exec, classicScript);
+    }
+  }
+
+  // This must be called also for compilation failure case, in order to
+  // dispatch test-only event.
+  rv = MaybePrepareForBytecodeEncodingAfterExecute(aRequest, rv);
+
+  // Even if we are not saving the bytecode of the current script, we have
+  // to trigger the encoding of the bytecode, as the current script can
+  // call functions of a script for which we are recording the bytecode.
+  LOG(("ScriptLoadRequest (%p): ScriptLoader = %p", aRequest, this));
+  MaybeTriggerBytecodeEncoding();
+
+  return rv;
+}
+
+/* static */
+LoadedScript* ScriptLoader::GetActiveScript(JSContext* aCx) {
+  JS::Value value = JS::GetScriptedCallerPrivate(aCx);
+  if (value.isUndefined()) {
+    return nullptr;
+  }
+
+  return static_cast<LoadedScript*>(value.toPrivate());
+}
+
+void ScriptLoader::RegisterForBytecodeEncoding(ScriptLoadRequest* aRequest) {
+  MOZ_ASSERT(aRequest->mCacheInfo);
+  MOZ_ASSERT(aRequest->IsMarkedForBytecodeEncoding());
+  MOZ_DIAGNOSTIC_ASSERT(!aRequest->isInList());
+  mBytecodeEncodingQueue.AppendElement(aRequest);
+}
+
+void ScriptLoader::LoadEventFired() {
+  mLoadEventFired = true;
+  MaybeTriggerBytecodeEncoding();
+
+  if (!mMainThreadParseTime.IsZero()) {
+    glean::javascript_pageload::parse_time.AccumulateRawDuration(
+        mMainThreadParseTime);
+  }
+}
+
+void ScriptLoader::Destroy() {
+  if (mShutdownObserver) {
+    mShutdownObserver->Unregister();
+    mShutdownObserver = nullptr;
+  }
+
+  CancelAndClearScriptLoadRequests();
+  GiveUpBytecodeEncoding();
+}
+
+void ScriptLoader::MaybeTriggerBytecodeEncoding() {
+  // If we already gave up, ensure that we are not going to enqueue any script,
+  // and that we finalize them properly.
+  if (mGiveUpEncoding) {
+    LOG(("ScriptLoader (%p): Keep giving-up bytecode encoding.", this));
+    GiveUpBytecodeEncoding();
+    return;
+  }
+
+  // We wait for the load event to be fired before saving the bytecode of
+  // any script to the cache. It is quite common to have load event
+  // listeners trigger more JavaScript execution, that we want to save as
+  // part of this start-up bytecode cache.
+  if (!mLoadEventFired) {
+    LOG(("ScriptLoader (%p): Wait for the load-end event to fire.", this));
+    return;
+  }
+
+  // No need to fire any event if there is no bytecode to be saved.
+  if (mBytecodeEncodingQueue.isEmpty()) {
+    LOG(("ScriptLoader (%p): No script in queue to be encoded.", this));
+    return;
+  }
+
+  // Wait until all scripts are loaded before saving the bytecode, such that
+  // we capture most of the intialization of the page.
+  if (HasPendingRequests()) {
+    LOG(("ScriptLoader (%p): Wait for other pending request to finish.", this));
+    return;
+  }
+
+  // Create a new runnable dedicated to encoding the content of the bytecode of
+  // all enqueued scripts when the document is idle. In case of failure, we
+  // give-up on encoding the bytecode.
+  nsCOMPtr<nsIRunnable> encoder = NewRunnableMethod(
+      "ScriptLoader::EncodeBytecode", this, &ScriptLoader::EncodeBytecode);
+  if (NS_FAILED(NS_DispatchToCurrentThreadQueue(encoder.forget(),
+                                                EventQueuePriority::Idle))) {
+    GiveUpBytecodeEncoding();
+    return;
+  }
+
+  LOG(("ScriptLoader (%p): Schedule bytecode encoding.", this));
+}
+
+void ScriptLoader::EncodeBytecode() {
+  LOG(("ScriptLoader (%p): Start bytecode encoding.", this));
+
+  // If any script got added in the previous loop cycle, wait until all
+  // remaining script executions are completed, such that we capture most of
+  // the initialization.
+  if (HasPendingRequests()) {
+    return;
+  }
+
+  // Should not be encoding modules at all.
+  nsCOMPtr<nsIScriptGlobalObject> globalObject = GetScriptGlobalObject();
+  if (!globalObject) {
+    GiveUpBytecodeEncoding();
+    return;
+  }
+
+  nsCOMPtr<nsIScriptContext> context = globalObject->GetScriptContext();
+  if (!context) {
+    GiveUpBytecodeEncoding();
+    return;
+  }
+
+  AutoEntryScript aes(globalObject, "encode bytecode", true);
+  RefPtr<ScriptLoadRequest> request;
+  while (!mBytecodeEncodingQueue.isEmpty()) {
+    request = mBytecodeEncodingQueue.StealFirst();
+    MOZ_ASSERT(!IsWebExtensionRequest(request),
+               "Bytecode for web extension content scrips is not cached");
+    EncodeRequestBytecode(aes.cx(), request);
+    request->DropBytecode();
+    request->DropBytecodeCacheReferences();
+  }
+}
+
+void ScriptLoader::EncodeRequestBytecode(JSContext* aCx,
+                                         ScriptLoadRequest* aRequest) {
+  using namespace mozilla::Telemetry;
+  nsresult rv = NS_OK;
+  MOZ_ASSERT(aRequest->mCacheInfo);
+  auto bytecodeFailed = mozilla::MakeScopeExit([&]() {
+    TRACE_FOR_TEST_NONE(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                        "scriptloader_bytecode_failed");
+  });
+
+  bool result;
+  if (aRequest->IsModuleRequest()) {
+    ModuleScript* moduleScript = aRequest->AsModuleRequest()->mModuleScript;
+    JS::Rooted<JSObject*> module(aCx, moduleScript->ModuleRecord());
+    result =
+        JS::FinishIncrementalEncoding(aCx, module, aRequest->SRIAndBytecode());
+  } else {
+    JS::Rooted<JSScript*> script(aCx, aRequest->mScriptForBytecodeEncoding);
+    result =
+        JS::FinishIncrementalEncoding(aCx, script, aRequest->SRIAndBytecode());
+  }
+  if (!result) {
+    // Encoding can be aborted for non-supported syntax (e.g. asm.js), or
+    // any other internal error.
+    // We don't care the error and just give up encoding.
+    JS_ClearPendingException(aCx);
+
+    LOG(("ScriptLoadRequest (%p): Cannot serialize bytecode", aRequest));
+    return;
+  }
+
+  Vector<uint8_t> compressedBytecode;
+  // TODO probably need to move this to a helper thread
+  if (!ScriptBytecodeCompress(aRequest->SRIAndBytecode(),
+                              aRequest->GetSRILength(), compressedBytecode)) {
+    return;
+  }
+
+  if (compressedBytecode.length() >= UINT32_MAX) {
+    LOG(
+        ("ScriptLoadRequest (%p): Bytecode cache is too large to be decoded "
+         "correctly.",
+         aRequest));
+    return;
+  }
+
+  // Open the output stream to the cache entry alternate data storage. This
+  // might fail if the stream is already open by another request, in which
+  // case, we just ignore the current one.
+  nsCOMPtr<nsIAsyncOutputStream> output;
+  rv = aRequest->mCacheInfo->OpenAlternativeOutputStream(
+      BytecodeMimeTypeFor(aRequest),
+      static_cast<int64_t>(compressedBytecode.length()),
+      getter_AddRefs(output));
+  if (NS_FAILED(rv)) {
+    LOG(
+        ("ScriptLoadRequest (%p): Cannot open bytecode cache (rv = %X, output "
+         "= %p)",
+         aRequest, unsigned(rv), output.get()));
+    return;
+  }
+  MOZ_ASSERT(output);
+
+  auto closeOutStream = mozilla::MakeScopeExit([&]() {
+    rv = output->CloseWithStatus(rv);
+    LOG(("ScriptLoadRequest (%p): Closing (rv = %X)", aRequest, unsigned(rv)));
+  });
+
+  uint32_t n;
+  rv = output->Write(reinterpret_cast<char*>(compressedBytecode.begin()),
+                     compressedBytecode.length(), &n);
+  LOG(
+      ("ScriptLoadRequest (%p): Write bytecode cache (rv = %X, length = %u, "
+       "written = %u)",
+       aRequest, unsigned(rv), unsigned(compressedBytecode.length()), n));
+  if (NS_FAILED(rv)) {
+    return;
+  }
+
+  MOZ_RELEASE_ASSERT(compressedBytecode.length() == n);
+
+  bytecodeFailed.release();
+  TRACE_FOR_TEST_NONE(aRequest->GetScriptLoadContext()->GetScriptElement(),
+                      "scriptloader_bytecode_saved");
+}
+
+void ScriptLoader::GiveUpBytecodeEncoding() {
+  // If the document went away prematurely, we still want to set this, in order
+  // to avoid queuing more scripts.
+  mGiveUpEncoding = true;
+
+  // Ideally we prefer to properly end the incremental encoder, such that we
+  // would not keep a large buffer around.  If we cannot, we fallback on the
+  // removal of all request from the current list and these large buffers would
+  // be removed at the same time as the source object.
+  nsCOMPtr<nsIScriptGlobalObject> globalObject = GetScriptGlobalObject();
+  AutoAllowLegacyScriptExecution exemption;
+  Maybe<AutoEntryScript> aes;
+
+  if (globalObject) {
+    nsCOMPtr<nsIScriptContext> context = globalObject->GetScriptContext();
+    if (context) {
+      aes.emplace(globalObject, "give-up bytecode encoding", true);
+    }
+  }
+
+  while (!mBytecodeEncodingQueue.isEmpty()) {
+    RefPtr<ScriptLoadRequest> request = mBytecodeEncodingQueue.StealFirst();
+    LOG(("ScriptLoadRequest (%p): Cannot serialize bytecode", request.get()));
+    TRACE_FOR_TEST_NONE(request->GetScriptLoadContext()->GetScriptElement(),
+                        "scriptloader_bytecode_failed");
+    MOZ_ASSERT(!IsWebExtensionRequest(request));
+
+    if (aes.isSome()) {
+      if (request->IsModuleRequest()) {
+        ModuleScript* moduleScript = request->AsModuleRequest()->mModuleScript;
+        JS::Rooted<JSObject*> module(aes->cx(), moduleScript->ModuleRecord());
+        JS::AbortIncrementalEncoding(module);
+      } else {
+        JS::Rooted<JSScript*> script(aes->cx(),
+                                     request->mScriptForBytecodeEncoding);
+        JS::AbortIncrementalEncoding(script);
+      }
+    }
+
+    request->DropBytecode();
+    request->DropBytecodeCacheReferences();
+  }
+}
+
+bool ScriptLoader::HasPendingRequests() const {
+  return mParserBlockingRequest || !mXSLTRequests.isEmpty() ||
+         !mLoadedAsyncRequests.isEmpty() ||
+         !mNonAsyncExternalScriptInsertedRequests.isEmpty() ||
+         !mDeferRequests.isEmpty() || HasPendingDynamicImports() ||
+         !mPendingChildLoaders.IsEmpty();
+  // mOffThreadCompilingRequests are already being processed.
+}
+
+bool ScriptLoader::HasPendingDynamicImports() const {
+  if (mModuleLoader && mModuleLoader->HasPendingDynamicImports()) {
+    return true;
+  }
+
+  for (ModuleLoader* loader : mWebExtModuleLoaders) {
+    if (loader->HasPendingDynamicImports()) {
+      return true;
+    }
+  }
+
+  for (ModuleLoader* loader : mShadowRealmModuleLoaders) {
+    if (loader->HasPendingDynamicImports()) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+void ScriptLoader::ProcessPendingRequestsAsync() {
+  if (HasPendingRequests()) {
+    nsCOMPtr<nsIRunnable> task =
+        NewRunnableMethod("dom::ScriptLoader::ProcessPendingRequests", this,
+                          &ScriptLoader::ProcessPendingRequests);
+    if (mDocument) {
+      mDocument->Dispatch(task.forget());
+    } else {
+      NS_DispatchToCurrentThread(task.forget());
+    }
+  }
+}
+
+void ScriptLoader::ProcessPendingRequests() {
+  RefPtr<ScriptLoadRequest> request;
+
+  if (mParserBlockingRequest && mParserBlockingRequest->IsFinished() &&
+      ReadyToExecuteParserBlockingScripts()) {
+    request.swap(mParserBlockingRequest);
+    UnblockParser(request);
+    ProcessRequest(request);
+    ContinueParserAsync(request);
+  }
+
+  while (ReadyToExecuteParserBlockingScripts() && !mXSLTRequests.isEmpty() &&
+         mXSLTRequests.getFirst()->IsFinished()) {
+    request = mXSLTRequests.StealFirst();
+    ProcessRequest(request);
+  }
+
+  while (ReadyToExecuteScripts() && !mLoadedAsyncRequests.isEmpty()) {
+    request = mLoadedAsyncRequests.StealFirst();
+    if (request->IsModuleRequest()) {
+      ProcessRequest(request);
+    } else {
+      CompileOffThreadOrProcessRequest(request);
+    }
+  }
+
+  while (ReadyToExecuteScripts() &&
+         !mNonAsyncExternalScriptInsertedRequests.isEmpty() &&
+         mNonAsyncExternalScriptInsertedRequests.getFirst()->IsFinished()) {
+    // Violate the HTML5 spec and execute these in the insertion order in
+    // order to make LABjs and the "order" plug-in for RequireJS work with
+    // their Gecko-sniffed code path. See
+    // http://lists.w3.org/Archives/Public/public-html/2010Oct/0088.html
+    request = mNonAsyncExternalScriptInsertedRequests.StealFirst();
+    ProcessRequest(request);
+  }
+
+  if (mDeferCheckpointReached && mXSLTRequests.isEmpty()) {
+    while (ReadyToExecuteScripts() && !mDeferRequests.isEmpty() &&
+           mDeferRequests.getFirst()->IsFinished()) {
+      request = mDeferRequests.StealFirst();
+      ProcessRequest(request);
+    }
+  }
+
+  while (!mPendingChildLoaders.IsEmpty() &&
+         ReadyToExecuteParserBlockingScripts()) {
+    RefPtr<ScriptLoader> child = mPendingChildLoaders[0];
+    mPendingChildLoaders.RemoveElementAt(0);
+    child->RemoveParserBlockingScriptExecutionBlocker();
+  }
+
+  if (mDeferCheckpointReached && mDocument && !mParserBlockingRequest &&
+      mNonAsyncExternalScriptInsertedRequests.isEmpty() &&
+      mXSLTRequests.isEmpty() && mDeferRequests.isEmpty() &&
+      MaybeRemovedDeferRequests()) {
+    return ProcessPendingRequests();
+  }
+
+  if (mDeferCheckpointReached && mDocument && !mParserBlockingRequest &&
+      mLoadingAsyncRequests.isEmpty() && mLoadedAsyncRequests.isEmpty() &&
+      mNonAsyncExternalScriptInsertedRequests.isEmpty() &&
+      mXSLTRequests.isEmpty() && mDeferRequests.isEmpty()) {
+    // No more pending scripts; time to unblock onload.
+    // OK to unblock onload synchronously here, since callers must be
+    // prepared for the world changing anyway.
+    mDeferCheckpointReached = false;
+    mDocument->UnblockOnload(true);
+  }
+}
+
+bool ScriptLoader::ReadyToExecuteParserBlockingScripts() {
+  // Make sure the SelfReadyToExecuteParserBlockingScripts check is first, so
+  // that we don't block twice on an ancestor.
+  if (!SelfReadyToExecuteParserBlockingScripts()) {
+    return false;
+  }
+
+  if (mDocument && mDocument->GetWindowContext()) {
+    for (WindowContext* wc =
+             mDocument->GetWindowContext()->GetParentWindowContext();
+         wc; wc = wc->GetParentWindowContext()) {
+      if (Document* doc = wc->GetDocument()) {
+        ScriptLoader* ancestor = doc->ScriptLoader();
+        if (!ancestor->SelfReadyToExecuteParserBlockingScripts() &&
+            ancestor->AddPendingChildLoader(this)) {
+          AddParserBlockingScriptExecutionBlocker();
+          return false;
+        }
+      }
+    }
+  }
+
+  return true;
+}
+
+template <typename Unit>
+static nsresult ConvertToUnicode(nsIChannel* aChannel, const uint8_t* aData,
+                                 uint32_t aLength,
+                                 const nsAString& aHintCharset,
+                                 Document* aDocument, Unit*& aBufOut,
+                                 size_t& aLengthOut) {
+  if (!aLength) {
+    aBufOut = nullptr;
+    aLengthOut = 0;
+    return NS_OK;
+  }
+
+  auto data = Span(aData, aLength);
+
+  // The encoding info precedence is as follows from high to low:
+  // The BOM
+  // HTTP Content-Type (if name recognized)
+  // charset attribute (if name recognized)
+  // The encoding of the document
+
+  UniquePtr<Decoder> unicodeDecoder;
+
+  const Encoding* encoding;
+  std::tie(encoding, std::ignore) = Encoding::ForBOM(data);
+  if (encoding) {
+    unicodeDecoder = encoding->NewDecoderWithBOMRemoval();
+  }
+
+  if (!unicodeDecoder && aChannel) {
+    nsAutoCString label;
+    if (NS_SUCCEEDED(aChannel->GetContentCharset(label)) &&
+        (encoding = Encoding::ForLabel(label))) {
+      unicodeDecoder = encoding->NewDecoderWithoutBOMHandling();
+    }
+  }
+
+  if (!unicodeDecoder && (encoding = Encoding::ForLabel(aHintCharset))) {
+    unicodeDecoder = encoding->NewDecoderWithoutBOMHandling();
+  }
+
+  if (!unicodeDecoder && aDocument) {
+    unicodeDecoder =
+        aDocument->GetDocumentCharacterSet()->NewDecoderWithoutBOMHandling();
+  }
+
+  if (!unicodeDecoder) {
+    // Curiously, there are various callers that don't pass aDocument. The
+    // fallback in the old code was ISO-8859-1, which behaved like
+    // windows-1252.
+    unicodeDecoder = WINDOWS_1252_ENCODING->NewDecoderWithoutBOMHandling();
+  }
+
+  auto signalOOM = mozilla::MakeScopeExit([&aBufOut, &aLengthOut]() {
+    aBufOut = nullptr;
+    aLengthOut = 0;
+  });
+
+  CheckedInt<size_t> bufferLength =
+      ScriptDecoding<Unit>::MaxBufferLength(unicodeDecoder, aLength);
+  if (!bufferLength.isValid()) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  CheckedInt<size_t> bufferByteSize = bufferLength * sizeof(Unit);
+  if (!bufferByteSize.isValid()) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  aBufOut = static_cast<Unit*>(js_malloc(bufferByteSize.value()));
+  if (!aBufOut) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  signalOOM.release();
+  aLengthOut = ScriptDecoding<Unit>::DecodeInto(
+      unicodeDecoder, data, Span(aBufOut, bufferLength.value()),
+      /* aEndOfSource = */ true);
+  return NS_OK;
+}
+
+/* static */
+nsresult ScriptLoader::ConvertToUTF16(
+    nsIChannel* aChannel, const uint8_t* aData, uint32_t aLength,
+    const nsAString& aHintCharset, Document* aDocument,
+    UniquePtr<char16_t[], JS::FreePolicy>& aBufOut, size_t& aLengthOut) {
+  char16_t* bufOut;
+  nsresult rv = ConvertToUnicode(aChannel, aData, aLength, aHintCharset,
+                                 aDocument, bufOut, aLengthOut);
+  if (NS_SUCCEEDED(rv)) {
+    aBufOut.reset(bufOut);
+  }
+  return rv;
+}
+
+/* static */
+nsresult ScriptLoader::ConvertToUTF8(
+    nsIChannel* aChannel, const uint8_t* aData, uint32_t aLength,
+    const nsAString& aHintCharset, Document* aDocument,
+    UniquePtr<Utf8Unit[], JS::FreePolicy>& aBufOut, size_t& aLengthOut) {
+  Utf8Unit* bufOut;
+  nsresult rv = ConvertToUnicode(aChannel, aData, aLength, aHintCharset,
+                                 aDocument, bufOut, aLengthOut);
+  if (NS_SUCCEEDED(rv)) {
+    aBufOut.reset(bufOut);
+  }
+  return rv;
+}
+
+nsresult ScriptLoader::OnStreamComplete(
+    nsIIncrementalStreamLoader* aLoader, ScriptLoadRequest* aRequest,
+    nsresult aChannelStatus, nsresult aSRIStatus,
+    SRICheckDataVerifier* aSRIDataVerifier) {
+  NS_ASSERTION(aRequest, "null request in stream complete handler");
+  NS_ENSURE_TRUE(aRequest, NS_ERROR_FAILURE);
+
+  nsresult rv = VerifySRI(aRequest, aLoader, aSRIStatus, aSRIDataVerifier);
+
+  if (NS_SUCCEEDED(rv)) {
+    // If we are loading from source, save the computed SRI hash or a dummy SRI
+    // hash in case we are going to save the bytecode of this script in the
+    // cache.
+    if (aRequest->IsSource()) {
+      uint32_t sriLength = 0;
+      rv = SaveSRIHash(aRequest, aSRIDataVerifier, &sriLength);
+      JS::TranscodeBuffer& bytecode = aRequest->SRIAndBytecode();
+      MOZ_ASSERT_IF(NS_SUCCEEDED(rv), bytecode.length() == sriLength);
+
+      // TODO: (Bug 1800896) This code should be moved into SaveSRIHash, and the
+      // SRI out-param can be removed.
+      aRequest->SetSRILength(sriLength);
+      if (aRequest->GetSRILength() != sriLength) {
+        // The bytecode is aligned in the bytecode buffer, and space might be
+        // reserved for padding after the SRI hash.
+        if (!bytecode.resize(aRequest->GetSRILength())) {
+          return NS_ERROR_OUT_OF_MEMORY;
+        }
+      }
+    }
+
+    if (NS_SUCCEEDED(rv)) {
+      rv = PrepareLoadedRequest(aRequest, aLoader, aChannelStatus);
+    }
+
+    if (NS_FAILED(rv)) {
+      ReportErrorToConsole(aRequest, rv);
+    }
+  }
+
+  if (NS_FAILED(rv)) {
+    // When loading bytecode, we verify the SRI hash. If it does not match the
+    // one from the document we restart the load, forcing us to load the source
+    // instead. If this happens do not remove the current request from script
+    // loader's data structures or fire any events.
+    if (aChannelStatus != NS_BINDING_RETARGETED) {
+      HandleLoadError(aRequest, rv);
+    }
+  }
+
+  // Process our request and/or any pending ones
+  ProcessPendingRequests();
+
+  return rv;
+}
+
+nsresult ScriptLoader::VerifySRI(ScriptLoadRequest* aRequest,
+                                 nsIIncrementalStreamLoader* aLoader,
+                                 nsresult aSRIStatus,
+                                 SRICheckDataVerifier* aSRIDataVerifier) const {
+  nsCOMPtr<nsIRequest> channelRequest;
+  aLoader->GetRequest(getter_AddRefs(channelRequest));
+  nsCOMPtr<nsIChannel> channel;
+  channel = do_QueryInterface(channelRequest);
+
+  nsresult rv = NS_OK;
+  if (!aRequest->mIntegrity.IsEmpty() && NS_SUCCEEDED((rv = aSRIStatus))) {
+    MOZ_ASSERT(aSRIDataVerifier);
+    MOZ_ASSERT(mReporter);
+
+    nsAutoCString sourceUri;
+    if (mDocument && mDocument->GetDocumentURI()) {
+      mDocument->GetDocumentURI()->GetAsciiSpec(sourceUri);
+    }
+    rv = aSRIDataVerifier->Verify(aRequest->mIntegrity, channel, sourceUri,
+                                  mReporter);
+    if (channelRequest) {
+      mReporter->FlushReportsToConsole(
+          nsContentUtils::GetInnerWindowID(channelRequest));
+    } else {
+      mReporter->FlushConsoleReports(mDocument);
+    }
+    if (NS_FAILED(rv)) {
+      rv = NS_ERROR_SRI_CORRUPT;
+    }
+  }
+
+  return rv;
+}
+
+nsresult ScriptLoader::SaveSRIHash(ScriptLoadRequest* aRequest,
+                                   SRICheckDataVerifier* aSRIDataVerifier,
+                                   uint32_t* sriLength) const {
+  MOZ_ASSERT(aRequest->IsSource());
+  JS::TranscodeBuffer& bytecode = aRequest->SRIAndBytecode();
+  MOZ_ASSERT(bytecode.empty());
+
+  uint32_t len;
+
+  // If the integrity metadata does not correspond to a valid hash function,
+  // IsComplete would be false.
+  if (!aRequest->mIntegrity.IsEmpty() && aSRIDataVerifier->IsComplete()) {
+    MOZ_ASSERT(bytecode.length() == 0);
+
+    // Encode the SRI computed hash.
+    len = aSRIDataVerifier->DataSummaryLength();
+
+    if (!bytecode.resize(len)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    DebugOnly<nsresult> res =
+        aSRIDataVerifier->ExportDataSummary(len, bytecode.begin());
+    MOZ_ASSERT(NS_SUCCEEDED(res));
+  } else {
+    MOZ_ASSERT(bytecode.length() == 0);
+
+    // Encode a dummy SRI hash.
+    len = SRICheckDataVerifier::EmptyDataSummaryLength();
+
+    if (!bytecode.resize(len)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    DebugOnly<nsresult> res =
+        SRICheckDataVerifier::ExportEmptyDataSummary(len, bytecode.begin());
+    MOZ_ASSERT(NS_SUCCEEDED(res));
+  }
+
+  // Verify that the exported and predicted length correspond.
+  DebugOnly<uint32_t> srilen{};
+  MOZ_ASSERT(NS_SUCCEEDED(
+      SRICheckDataVerifier::DataSummaryLength(len, bytecode.begin(), &srilen)));
+  MOZ_ASSERT(srilen == len);
+
+  *sriLength = len;
+
+  return NS_OK;
+}
+
+void ScriptLoader::ReportErrorToConsole(ScriptLoadRequest* aRequest,
+                                        nsresult aResult) const {
+  MOZ_ASSERT(aRequest);
+
+  if (aRequest->GetScriptLoadContext()->IsPreload()) {
+    // Skip reporting errors in preload requests. If the request is actually
+    // used then we will report the error in ReportPreloadErrorsToConsole below.
+    aRequest->GetScriptLoadContext()->mUnreportedPreloadError = aResult;
+    return;
+  }
+
+  bool isScript = !aRequest->IsModuleRequest();
+  const char* message;
+  if (aResult == NS_ERROR_MALFORMED_URI) {
+    message = isScript ? "ScriptSourceMalformed" : "ModuleSourceMalformed";
+  } else if (aResult == NS_ERROR_DOM_BAD_URI) {
+    message = isScript ? "ScriptSourceNotAllowed" : "ModuleSourceNotAllowed";
+  } else if (aResult == NS_ERROR_DOM_WEBEXT_CONTENT_SCRIPT_URI) {
+    MOZ_ASSERT(!isScript);
+    message = "WebExtContentScriptModuleSourceNotAllowed";
+  } else if (net::UrlClassifierFeatureFactory::IsClassifierBlockingErrorCode(
+                 aResult)) {
+    // Blocking classifier error codes already show their own console messages.
+    return;
+  } else {
+    message = isScript ? "ScriptSourceLoadFailed" : "ModuleSourceLoadFailed";
+  }
+
+  AutoTArray<nsString, 1> params;
+  CopyUTF8toUTF16(aRequest->mURI->GetSpecOrDefault(), *params.AppendElement());
+
+  nsIScriptElement* element =
+      aRequest->GetScriptLoadContext()->GetScriptElement();
+  uint32_t lineNo = element ? element->GetScriptLineNumber() : 0;
+  JS::ColumnNumberOneOrigin columnNo;
+  if (element) {
+    columnNo = element->GetScriptColumnNumber();
+  }
+
+  nsContentUtils::ReportToConsole(
+      nsIScriptError::warningFlag, "Script Loader"_ns, mDocument,
+      nsContentUtils::eDOM_PROPERTIES, message, params, nullptr, u""_ns, lineNo,
+      columnNo.oneOriginValue());
+}
+
+void ScriptLoader::ReportWarningToConsole(
+    ScriptLoadRequest* aRequest, const char* aMessageName,
+    const nsTArray<nsString>& aParams) const {
+  nsIScriptElement* element =
+      aRequest->GetScriptLoadContext()->GetScriptElement();
+  uint32_t lineNo = element ? element->GetScriptLineNumber() : 0;
+  JS::ColumnNumberOneOrigin columnNo;
+  if (element) {
+    columnNo = element->GetScriptColumnNumber();
+  }
+  nsContentUtils::ReportToConsole(
+      nsIScriptError::warningFlag, "Script Loader"_ns, mDocument,
+      nsContentUtils::eDOM_PROPERTIES, aMessageName, aParams, nullptr, u""_ns,
+      lineNo, columnNo.oneOriginValue());
+}
+
+void ScriptLoader::ReportPreloadErrorsToConsole(ScriptLoadRequest* aRequest) {
+  if (NS_FAILED(aRequest->GetScriptLoadContext()->mUnreportedPreloadError)) {
+    ReportErrorToConsole(
+        aRequest, aRequest->GetScriptLoadContext()->mUnreportedPreloadError);
+    aRequest->GetScriptLoadContext()->mUnreportedPreloadError = NS_OK;
+  }
+
+  if (aRequest->IsModuleRequest()) {
+    for (const auto& childRequest : aRequest->AsModuleRequest()->mImports) {
+      ReportPreloadErrorsToConsole(childRequest);
+    }
+  }
+}
+
+void ScriptLoader::HandleLoadError(ScriptLoadRequest* aRequest,
+                                   nsresult aResult) {
+  /*
+   * Handle script not loading error because source was an tracking URL (or
+   * fingerprinting, cryptomining, etc).
+   * We make a note of this script node by including it in a dedicated
+   * array of blocked tracking nodes under its parent document.
+   */
+  if (net::UrlClassifierFeatureFactory::IsClassifierBlockingErrorCode(
+          aResult)) {
+    nsCOMPtr<nsIContent> cont =
+        do_QueryInterface(aRequest->GetScriptLoadContext()->GetScriptElement());
+    mDocument->AddBlockedNodeByClassifier(cont);
+  }
+
+  if (aRequest->IsModuleRequest()) {
+    MOZ_ASSERT(!aRequest->GetScriptLoadContext()->mIsInline);
+    aRequest->AsModuleRequest()->OnFetchComplete(aResult);
+  }
+
+  if (aRequest->GetScriptLoadContext()->mInDeferList) {
+    MOZ_ASSERT_IF(aRequest->IsModuleRequest(),
+                  aRequest->AsModuleRequest()->IsTopLevel());
+    if (aRequest->isInList()) {
+      RefPtr<ScriptLoadRequest> req = mDeferRequests.Steal(aRequest);
+      FireScriptAvailable(aResult, req);
+    }
+  } else if (aRequest->GetScriptLoadContext()->mInAsyncList) {
+    MOZ_ASSERT_IF(aRequest->IsModuleRequest(),
+                  aRequest->AsModuleRequest()->IsTopLevel());
+    if (aRequest->isInList()) {
+      RefPtr<ScriptLoadRequest> req = mLoadingAsyncRequests.Steal(aRequest);
+      FireScriptAvailable(aResult, req);
+    }
+  } else if (aRequest->GetScriptLoadContext()->mIsNonAsyncScriptInserted) {
+    if (aRequest->isInList()) {
+      RefPtr<ScriptLoadRequest> req =
+          mNonAsyncExternalScriptInsertedRequests.Steal(aRequest);
+      FireScriptAvailable(aResult, req);
+    }
+  } else if (aRequest->GetScriptLoadContext()->mIsXSLT) {
+    if (aRequest->isInList()) {
+      RefPtr<ScriptLoadRequest> req = mXSLTRequests.Steal(aRequest);
+      FireScriptAvailable(aResult, req);
+    }
+  } else if (aRequest->GetScriptLoadContext()->IsPreload()) {
+    if (aRequest->IsModuleRequest()) {
+      aRequest->Cancel();
+    }
+    if (aRequest->IsTopLevel()) {
+      // Request may already have been removed by
+      // CancelAndClearScriptLoadRequests.
+      mPreloads.RemoveElement(aRequest, PreloadRequestComparator());
+    }
+    MOZ_ASSERT(!aRequest->isInList());
+    AccumulateCategorical(LABELS_DOM_SCRIPT_PRELOAD_RESULT::LoadError);
+  } else if (aRequest->IsModuleRequest()) {
+    ModuleLoadRequest* modReq = aRequest->AsModuleRequest();
+    if (modReq->IsDynamicImport()) {
+      MOZ_ASSERT(modReq->IsTopLevel());
+      if (aRequest->isInList()) {
+        modReq->CancelDynamicImport(aResult);
+      }
+    } else {
+      MOZ_ASSERT(!modReq->isInList());
+      modReq->Cancel();
+    }
+  } else if (mParserBlockingRequest == aRequest) {
+    MOZ_ASSERT(!aRequest->isInList());
+    mParserBlockingRequest = nullptr;
+    UnblockParser(aRequest);
+
+    // Ensure that we treat aRequest->GetScriptLoadContext()->GetScriptElement()
+    // as our current parser-inserted script while firing onerror on it.
+    MOZ_ASSERT(aRequest->GetScriptLoadContext()
+                   ->GetScriptElement()
+                   ->GetParserCreated());
+    nsCOMPtr<nsIScriptElement> oldParserInsertedScript =
+        mCurrentParserInsertedScript;
+    mCurrentParserInsertedScript =
+        aRequest->GetScriptLoadContext()->GetScriptElement();
+    FireScriptAvailable(aResult, aRequest);
+    ContinueParserAsync(aRequest);
+    mCurrentParserInsertedScript = oldParserInsertedScript;
+  } else {
+    // This happens for blocking requests cancelled by ParsingComplete().
+    // Ignore cancellation status for link-preload requests, as cancellation can
+    // be omitted for them when SRI is stronger on consumer tags.
+    MOZ_ASSERT(aRequest->IsCanceled() ||
+               aRequest->GetScriptLoadContext()->IsLinkPreloadScript());
+    MOZ_ASSERT(!aRequest->isInList());
+  }
+}
+
+void ScriptLoader::UnblockParser(ScriptLoadRequest* aParserBlockingRequest) {
+  aParserBlockingRequest->GetScriptLoadContext()
+      ->GetScriptElement()
+      ->UnblockParser();
+}
+
+void ScriptLoader::ContinueParserAsync(
+    ScriptLoadRequest* aParserBlockingRequest) {
+  aParserBlockingRequest->GetScriptLoadContext()
+      ->GetScriptElement()
+      ->ContinueParserAsync();
+}
+
+uint32_t ScriptLoader::NumberOfProcessors() {
+  if (mNumberOfProcessors > 0) {
+    return mNumberOfProcessors;
+  }
+
+  int32_t numProcs = PR_GetNumberOfProcessors();
+  if (numProcs > 0) {
+    mNumberOfProcessors = numProcs;
+  }
+  return mNumberOfProcessors;
+}
+
+int32_t ScriptLoader::PhysicalSizeOfMemoryInGB() {
+  // 0 is a valid result from PR_GetPhysicalMemorySize() which
+  // means a failure occured.
+  if (mPhysicalSizeOfMemory >= 0) {
+    return mPhysicalSizeOfMemory;
+  }
+
+  // Save the size in GB.
+  mPhysicalSizeOfMemory =
+      static_cast<int32_t>(PR_GetPhysicalMemorySize() >> 30);
+  return mPhysicalSizeOfMemory;
+}
+
+bool ScriptLoader::ShouldApplyDelazifyStrategy(ScriptLoadRequest* aRequest) {
+  // Full parse everything if negative.
+  if (StaticPrefs::dom_script_loader_delazification_max_size() < 0) {
+    return true;
+  }
+
+  // Be conservative on machines with 2GB or less of memory.
+  if (PhysicalSizeOfMemoryInGB() <=
+      StaticPrefs::dom_script_loader_delazification_min_mem()) {
+    return false;
+  }
+
+  uint32_t max_size = static_cast<uint32_t>(
+      StaticPrefs::dom_script_loader_delazification_max_size());
+  uint32_t script_size =
+      aRequest->ScriptTextLength() > 0
+          ? static_cast<uint32_t>(aRequest->ScriptTextLength())
+          : 0;
+
+  if (mTotalFullParseSize + script_size < max_size) {
+    return true;
+  }
+
+  if (LOG_ENABLED()) {
+    nsCString url = aRequest->mURI->GetSpecOrDefault();
+    LOG(
+        ("ScriptLoadRequest (%p): non-on-demand-only Parsing Disabled for (%s) "
+         "with size=%u because mTotalFullParseSize=%u would exceed max_size=%u",
+         aRequest, url.get(), script_size, mTotalFullParseSize, max_size));
+  }
+
+  return false;
+}
+
+void ScriptLoader::ApplyDelazifyStrategy(JS::CompileOptions* aOptions) {
+  JS::DelazificationOption strategy =
+      JS::DelazificationOption::ParseEverythingEagerly;
+  uint32_t strategyIndex =
+      StaticPrefs::dom_script_loader_delazification_strategy();
+
+  // Assert that all enumerated values of DelazificationOption are dense between
+  // OnDemandOnly and ParseEverythingEagerly.
+#ifdef DEBUG
+  uint32_t count = 0;
+  uint32_t mask = 0;
+#  define _COUNT_ENTRIES(Name) count++;
+#  define _MASK_ENTRIES(Name) \
+    mask |= 1 << uint32_t(JS::DelazificationOption::Name);
+
+  FOREACH_DELAZIFICATION_STRATEGY(_COUNT_ENTRIES);
+  MOZ_ASSERT(count == uint32_t(strategy) + 1);
+  FOREACH_DELAZIFICATION_STRATEGY(_MASK_ENTRIES);
+  MOZ_ASSERT(((mask + 1) & mask) == 0);
+#  undef _COUNT_ENTRIES
+#  undef _MASK_ENTRIES
+#endif
+
+  // Any strategy index larger than ParseEverythingEagerly would default to
+  // ParseEverythingEagerly.
+  if (strategyIndex <= uint32_t(strategy)) {
+    strategy = JS::DelazificationOption(uint8_t(strategyIndex));
+  }
+
+  aOptions->setEagerDelazificationStrategy(strategy);
+}
+
+bool ScriptLoader::ShouldCompileOffThread(ScriptLoadRequest* aRequest) {
+  if (NumberOfProcessors() <= 1) {
+    return false;
+  }
+  if (aRequest == mParserBlockingRequest) {
+    return true;
+  }
+  if (SpeculativeOMTParsingEnabled()) {
+    // Processing non async inserted scripts too early can potentially delay the
+    // load event from firing so focus on other scripts instead.
+    if (aRequest->GetScriptLoadContext()->mIsNonAsyncScriptInserted &&
+        !StaticPrefs::
+            dom_script_loader_external_scripts_speculate_non_parser_inserted_enabled()) {
+      return false;
+    }
+
+    // Async and link preload scripts do not need to be parsed right away.
+    if (aRequest->GetScriptLoadContext()->IsAsyncScript() &&
+        !StaticPrefs::
+            dom_script_loader_external_scripts_speculate_async_enabled()) {
+      return false;
+    }
+
+    if (aRequest->GetScriptLoadContext()->IsLinkPreloadScript() &&
+        !StaticPrefs::
+            dom_script_loader_external_scripts_speculate_link_preload_enabled()) {
+      return false;
+    }
+
+    return true;
+  }
+  return false;
+}
+
+nsresult ScriptLoader::PrepareLoadedRequest(ScriptLoadRequest* aRequest,
+                                            nsIIncrementalStreamLoader* aLoader,
+                                            nsresult aStatus) {
+  if (NS_FAILED(aStatus)) {
+    return aStatus;
+  }
+
+  if (aRequest->IsCanceled()) {
+    return NS_BINDING_ABORTED;
+  }
+  MOZ_ASSERT(aRequest->IsFetching());
+  CollectScriptTelemetry(aRequest);
+
+  // If we don't have a document, then we need to abort further
+  // evaluation.
+  if (!mDocument) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  // If the load returned an error page, then we need to abort
+  nsCOMPtr<nsIRequest> req;
+  nsresult rv = aLoader->GetRequest(getter_AddRefs(req));
+  NS_ASSERTION(req, "StreamLoader's request went away prematurely");
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCOMPtr<nsIHttpChannel> httpChannel = do_QueryInterface(req);
+  if (httpChannel) {
+    bool requestSucceeded;
+    rv = httpChannel->GetRequestSucceeded(&requestSucceeded);
+    if (NS_SUCCEEDED(rv) && !requestSucceeded) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+
+    if (aRequest->IsModuleRequest()) {
+      // https://html.spec.whatwg.org/multipage/webappapis.html#fetch-a-single-module-script
+      // Update script's referrer-policy if there's a Referrer-Policy header in
+      // the HTTP response.
+      ReferrerPolicy policy =
+          nsContentUtils::GetReferrerPolicyFromChannel(httpChannel);
+      if (policy != ReferrerPolicy::_empty) {
+        aRequest->UpdateReferrerPolicy(policy);
+      }
+    }
+
+    nsAutoCString sourceMapURL;
+    if (nsContentUtils::GetSourceMapURL(httpChannel, sourceMapURL)) {
+      aRequest->mSourceMapURL = Some(NS_ConvertUTF8toUTF16(sourceMapURL));
+    }
+
+    nsCOMPtr<nsIClassifiedChannel> classifiedChannel = do_QueryInterface(req);
+    MOZ_ASSERT(classifiedChannel);
+    if (classifiedChannel &&
+        classifiedChannel->IsThirdPartyTrackingResource()) {
+      aRequest->GetScriptLoadContext()->SetIsTracking();
+    }
+  }
+
+  nsCOMPtr<nsIChannel> channel = do_QueryInterface(req);
+  // If this load was subject to a CORS check, don't flag it with a separate
+  // origin principal, so that it will treat our document's principal as the
+  // origin principal.  Module loads always use CORS.
+  if (!aRequest->IsModuleRequest() && aRequest->CORSMode() == CORS_NONE) {
+    rv = nsContentUtils::GetSecurityManager()->GetChannelResultPrincipal(
+        channel, getter_AddRefs(aRequest->mOriginPrincipal));
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  // This assertion could fire errorously if we ran out of memory when
+  // inserting the request in the array. However it's an unlikely case
+  // so if you see this assertion it is likely something else that is
+  // wrong, especially if you see it more than once.
+  NS_ASSERTION(mDeferRequests.Contains(aRequest) ||
+                   mLoadingAsyncRequests.Contains(aRequest) ||
+                   mNonAsyncExternalScriptInsertedRequests.Contains(aRequest) ||
+                   mXSLTRequests.Contains(aRequest) ||
+                   (aRequest->IsModuleRequest() &&
+                    (aRequest->AsModuleRequest()->IsRegisteredDynamicImport() ||
+                     !aRequest->AsModuleRequest()->IsTopLevel())) ||
+                   mPreloads.Contains(aRequest, PreloadRequestComparator()) ||
+                   mParserBlockingRequest == aRequest,
+               "aRequest should be pending!");
+
+  nsCOMPtr<nsIURI> uri;
+  rv = channel->GetOriginalURI(getter_AddRefs(uri));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  aRequest->SetBaseURLFromChannelAndOriginalURI(channel, uri);
+
+  if (aRequest->IsModuleRequest()) {
+    ModuleLoadRequest* request = aRequest->AsModuleRequest();
+
+    // When loading a module, only responses with a JavaScript MIME type are
+    // acceptable.
+    nsAutoCString mimeType;
+    channel->GetContentType(mimeType);
+    NS_ConvertUTF8toUTF16 typeString(mimeType);
+    if (!nsContentUtils::IsJavascriptMIMEType(typeString)) {
+      return NS_ERROR_FAILURE;
+    }
+
+    // Attempt to compile off main thread.
+    bool couldCompile = false;
+    rv = AttemptOffThreadScriptCompile(request, &couldCompile);
+    NS_ENSURE_SUCCESS(rv, rv);
+    if (couldCompile) {
+      return NS_OK;
+    }
+
+    // Otherwise compile it right away and start fetching descendents.
+    return request->OnFetchComplete(NS_OK);
+  }
+
+  // The script is now loaded and ready to run.
+  aRequest->SetReady();
+
+  // If speculative parsing is enabled attempt to compile all
+  // external scripts off-main-thread.  Otherwise, only omt compile scripts
+  // blocking the parser.
+  if (ShouldCompileOffThread(aRequest)) {
+    MOZ_ASSERT(!aRequest->IsModuleRequest());
+    bool couldCompile = false;
+    nsresult rv = AttemptOffThreadScriptCompile(aRequest, &couldCompile);
+    NS_ENSURE_SUCCESS(rv, rv);
+    if (couldCompile) {
+      MOZ_ASSERT(aRequest->mState == ScriptLoadRequest::State::Compiling,
+                 "Request should be off-thread compiling now.");
+      return NS_OK;
+    }
+
+    // If off-thread compile was rejected, continue with regular processing.
+  }
+
+  MaybeMoveToLoadedList(aRequest);
+
+  return NS_OK;
+}
+
+void ScriptLoader::DeferCheckpointReached() {
+  if (mDeferEnabled) {
+    // Have to check because we apparently get ParsingComplete
+    // without BeginDeferringScripts in some cases
+    mDeferCheckpointReached = true;
+  }
+
+  mDeferEnabled = false;
+  ProcessPendingRequests();
+}
+
+void ScriptLoader::ParsingComplete(bool aTerminated) {
+  if (aTerminated) {
+    CancelAndClearScriptLoadRequests();
+
+    // Have to call this even if aTerminated so we'll correctly unblock onload.
+    DeferCheckpointReached();
+  }
+}
+
+void ScriptLoader::PreloadURI(
+    nsIURI* aURI, const nsAString& aCharset, const nsAString& aType,
+    const nsAString& aCrossOrigin, const nsAString& aNonce,
+    const nsAString& aFetchPriority, const nsAString& aIntegrity,
+    bool aScriptFromHead, bool aAsync, bool aDefer, bool aLinkPreload,
+    const ReferrerPolicy aReferrerPolicy, uint64_t aEarlyHintPreloaderId) {
+  NS_ENSURE_TRUE_VOID(mDocument);
+  // Check to see if scripts has been turned off.
+  if (!mEnabled || !mDocument->IsScriptEnabled()) {
+    return;
+  }
+
+  ScriptKind scriptKind = ScriptKind::eClassic;
+
+  static const char kASCIIWhitespace[] = "\t\n\f\r ";
+
+  nsAutoString type(aType);
+  type.Trim(kASCIIWhitespace);
+  if (type.LowerCaseEqualsASCII("module")) {
+    scriptKind = ScriptKind::eModule;
+  }
+
+  if (scriptKind == ScriptKind::eClassic && !aType.IsEmpty() &&
+      !nsContentUtils::IsJavascriptMIMEType(aType)) {
+    // Unknown type.  Don't load it.
+    return;
+  }
+
+  SRIMetadata sriMetadata;
+  GetSRIMetadata(aIntegrity, &sriMetadata);
+
+  const auto requestPriority = FetchPriorityToRequestPriority(
+      nsGenericHTMLElement::ToFetchPriority(aFetchPriority));
+
+  // For link type "modulepreload":
+  // https://html.spec.whatwg.org/multipage/links.html#link-type-modulepreload
+  // Step 11. Let options be a script fetch options whose cryptographic nonce is
+  // cryptographic nonce, integrity metadata is integrity metadata, parser
+  // metadata is "not-parser-inserted", credentials mode is credentials mode,
+  // referrer policy is referrer policy, and fetch priority is fetch priority.
+  //
+  // We treat speculative <script> loads as parser-inserted, because they
+  // come from a parser. This will also match how they should be treated
+  // as a normal load.
+  RefPtr<ScriptLoadRequest> request =
+      CreateLoadRequest(scriptKind, aURI, nullptr, mDocument->NodePrincipal(),
+                        Element::StringToCORSMode(aCrossOrigin), aNonce,
+                        requestPriority, sriMetadata, aReferrerPolicy,
+                        aLinkPreload ? ParserMetadata::NotParserInserted
+                                     : ParserMetadata::ParserInserted);
+  request->GetScriptLoadContext()->mIsInline = false;
+  request->GetScriptLoadContext()->mScriptFromHead = aScriptFromHead;
+  request->GetScriptLoadContext()->SetScriptMode(aDefer, aAsync, aLinkPreload);
+  request->GetScriptLoadContext()->SetIsPreloadRequest();
+  request->mEarlyHintPreloaderId = aEarlyHintPreloaderId;
+
+  if (LOG_ENABLED()) {
+    nsAutoCString url;
+    aURI->GetAsciiSpec(url);
+    LOG(("ScriptLoadRequest (%p): Created preload request for %s",
+         request.get(), url.get()));
+  }
+
+  nsAutoString charset(aCharset);
+  nsresult rv = StartLoad(request, Some(charset));
+  if (NS_FAILED(rv)) {
+    return;
+  }
+
+  PreloadInfo* pi = mPreloads.AppendElement();
+  pi->mRequest = request;
+  pi->mCharset = aCharset;
+}
+
+void ScriptLoader::AddDeferRequest(ScriptLoadRequest* aRequest) {
+  MOZ_ASSERT(aRequest->GetScriptLoadContext()->IsDeferredScript());
+  MOZ_ASSERT(!aRequest->GetScriptLoadContext()->mInDeferList &&
+             !aRequest->GetScriptLoadContext()->mInAsyncList);
+  MOZ_ASSERT(!aRequest->GetScriptLoadContext()->mInCompilingList);
+
+  aRequest->GetScriptLoadContext()->mInDeferList = true;
+  mDeferRequests.AppendElement(aRequest);
+  if (mDeferEnabled && aRequest == mDeferRequests.getFirst() && mDocument &&
+      !mBlockingDOMContentLoaded) {
+    MOZ_ASSERT(mDocument->GetReadyStateEnum() == Document::READYSTATE_LOADING);
+    mBlockingDOMContentLoaded = true;
+    mDocument->BlockDOMContentLoaded();
+  }
+}
+
+void ScriptLoader::AddAsyncRequest(ScriptLoadRequest* aRequest) {
+  MOZ_ASSERT(aRequest->GetScriptLoadContext()->IsAsyncScript());
+  MOZ_ASSERT(!aRequest->GetScriptLoadContext()->mInDeferList &&
+             !aRequest->GetScriptLoadContext()->mInAsyncList);
+  MOZ_ASSERT(!aRequest->GetScriptLoadContext()->mInCompilingList);
+
+  aRequest->GetScriptLoadContext()->mInAsyncList = true;
+  if (aRequest->IsFinished()) {
+    mLoadedAsyncRequests.AppendElement(aRequest);
+  } else {
+    mLoadingAsyncRequests.AppendElement(aRequest);
+  }
+}
+
+void ScriptLoader::MaybeMoveToLoadedList(ScriptLoadRequest* aRequest) {
+  MOZ_ASSERT(aRequest->IsFinished());
+  MOZ_ASSERT(aRequest->IsTopLevel());
+
+  // If it's async, move it to the loaded list.
+  // aRequest->GetScriptLoadContext()->mInAsyncList really _should_ be in a
+  // list, but the consequences if it's not are bad enough we want to avoid
+  // trying to move it if it's not.
+  if (aRequest->GetScriptLoadContext()->mInAsyncList) {
+    MOZ_ASSERT(aRequest->isInList());
+    if (aRequest->isInList()) {
+      RefPtr<ScriptLoadRequest> req = mLoadingAsyncRequests.Steal(aRequest);
+      mLoadedAsyncRequests.AppendElement(req);
+    }
+  } else if (aRequest->IsModuleRequest() &&
+             aRequest->AsModuleRequest()->IsDynamicImport()) {
+    // Process dynamic imports with async scripts.
+    MOZ_ASSERT(!aRequest->isInList());
+    mLoadedAsyncRequests.AppendElement(aRequest);
+  }
+}
+
+bool ScriptLoader::MaybeRemovedDeferRequests() {
+  if (mDeferRequests.isEmpty() && mDocument && mBlockingDOMContentLoaded) {
+    mBlockingDOMContentLoaded = false;
+    mDocument->UnblockDOMContentLoaded();
+    return true;
+  }
+  return false;
+}
+
+DocGroup* ScriptLoader::GetDocGroup() const { return mDocument->GetDocGroup(); }
+
+void ScriptLoader::BeginDeferringScripts() {
+  mDeferEnabled = true;
+  if (mDeferCheckpointReached) {
+    // We already completed a parse and were just waiting for some async
+    // scripts to load (and were already blocking the load event waiting for
+    // that to happen), when document.open() happened and now we're doing a
+    // new parse.  We shouldn't block the load event again, but _should_ reset
+    // mDeferCheckpointReached to false.  It'll get set to true again when the
+    // DeferCheckpointReached call that corresponds to this
+    // BeginDeferringScripts call happens (on document.close()), since we just
+    // set mDeferEnabled to true.
+    mDeferCheckpointReached = false;
+  } else {
+    if (mDocument) {
+      mDocument->BlockOnload();
+    }
+  }
+}
+
+nsAutoScriptLoaderDisabler::nsAutoScriptLoaderDisabler(Document* aDoc) {
+  mLoader = aDoc->ScriptLoader();
+  mWasEnabled = mLoader->GetEnabled();
+  if (mWasEnabled) {
+    mLoader->SetEnabled(false);
+  }
+}
+
+nsAutoScriptLoaderDisabler::~nsAutoScriptLoaderDisabler() {
+  if (mWasEnabled) {
+    mLoader->SetEnabled(true);
+  }
+}
+
+#undef TRACE_FOR_TEST
+#undef TRACE_FOR_TEST_BOOL
+#undef TRACE_FOR_TEST_NONE
+
+#undef LOG
+
+}  // namespace mozilla::dom
diff --git a/dom/script/ScriptLoader.h b/dom/script/ScriptLoader.h
new file mode 100644
index 0000000000..bdfd64b024
--- /dev/null
+++ b/dom/script/ScriptLoader.h
@@ -0,0 +1,788 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ScriptLoader_h
+#define mozilla_dom_ScriptLoader_h
+
+#include "js/TypeDecls.h"
+#include "js/Utility.h"  // JS::FreePolicy
+#include "js/loader/LoadedScript.h"
+#include "js/loader/ScriptKind.h"
+#include "js/loader/ScriptLoadRequest.h"
+#include "mozilla/dom/ScriptLoadContext.h"
+#include "nsCOMPtr.h"
+#include "nsRefPtrHashtable.h"
+#include "nsIScriptElement.h"
+#include "nsCOMArray.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsTArray.h"
+#include "nsILoadInfo.h"  // nsSecurityFlags
+#include "nsINode.h"
+#include "nsIObserver.h"
+#include "nsIScriptLoaderObserver.h"
+#include "nsURIHashKey.h"
+#include "mozilla/CORSMode.h"
+#include "mozilla/dom/JSExecutionContext.h"  // JSExecutionContext
+#include "ModuleLoader.h"
+#include "mozilla/MaybeOneOf.h"
+#include "mozilla/MozPromise.h"
+
+class nsCycleCollectionTraversalCallback;
+class nsIChannel;
+class nsIConsoleReportCollector;
+class nsIContent;
+class nsIIncrementalStreamLoader;
+class nsIPrincipal;
+class nsIScriptGlobalObject;
+class nsIURI;
+
+namespace JS {
+
+class CompileOptions;
+
+template <typename UnitT>
+class SourceText;
+
+namespace loader {
+
+class LoadedScript;
+class ScriptLoaderInterface;
+class ModuleLoadRequest;
+class ModuleScript;
+class ScriptLoadRequest;
+class ScriptLoadRequestList;
+
+enum class ParserMetadata;
+
+}  // namespace loader
+}  // namespace JS
+
+namespace mozilla {
+
+class LazyLogModule;
+union Utf8Unit;
+
+namespace dom {
+
+class AutoJSAPI;
+class DocGroup;
+class Document;
+class ModuleLoader;
+class SRICheckDataVerifier;
+class SRIMetadata;
+class ScriptLoadHandler;
+class ScriptLoadContext;
+class ScriptLoader;
+class ScriptRequestProcessor;
+
+enum class ReferrerPolicy : uint8_t;
+enum class RequestPriority : uint8_t;
+
+class AsyncCompileShutdownObserver final : public nsIObserver {
+  ~AsyncCompileShutdownObserver() { Unregister(); }
+
+ public:
+  explicit AsyncCompileShutdownObserver(ScriptLoader* aLoader)
+      : mScriptLoader(aLoader) {}
+
+  void OnShutdown();
+  void Unregister();
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+
+ private:
+  // Defined during registration in ScriptLoader constructor, and
+  // cleared during destructor, ScriptLoader::Destroy() or Shutdown.
+  ScriptLoader* mScriptLoader;
+};
+
+//////////////////////////////////////////////////////////////
+// Script loader implementation
+//////////////////////////////////////////////////////////////
+
+class ScriptLoader final : public JS::loader::ScriptLoaderInterface {
+  class MOZ_STACK_CLASS AutoCurrentScriptUpdater {
+   public:
+    AutoCurrentScriptUpdater(ScriptLoader* aScriptLoader,
+                             nsIScriptElement* aCurrentScript)
+        : mOldScript(aScriptLoader->mCurrentScript),
+          mScriptLoader(aScriptLoader) {
+      nsCOMPtr<nsINode> node = do_QueryInterface(aCurrentScript);
+      mScriptLoader->mCurrentScript =
+          node && !node->IsInShadowTree() ? aCurrentScript : nullptr;
+    }
+
+    ~AutoCurrentScriptUpdater() {
+      mScriptLoader->mCurrentScript.swap(mOldScript);
+    }
+
+   private:
+    nsCOMPtr<nsIScriptElement> mOldScript;
+    ScriptLoader* mScriptLoader;
+  };
+
+  friend class JS::loader::ModuleLoadRequest;
+  friend class ScriptRequestProcessor;
+  friend class ModuleLoader;
+  friend class ScriptLoadHandler;
+  friend class AutoCurrentScriptUpdater;
+
+ public:
+  using MaybeSourceText =
+      mozilla::MaybeOneOf<JS::SourceText<char16_t>, JS::SourceText<Utf8Unit>>;
+
+  explicit ScriptLoader(Document* aDocument);
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(ScriptLoader)
+
+  /**
+   * Called when the document that owns this script loader changes global. The
+   * argument is null when the document is detached from a window.
+   */
+  void SetGlobalObject(nsIGlobalObject* aGlobalObject);
+
+  /**
+   * The loader maintains a weak reference to the document with
+   * which it is initialized. This call forces the reference to
+   * be dropped.
+   */
+  void DropDocumentReference() { mDocument = nullptr; }
+
+  /**
+   * Add an observer for all scripts loaded through this loader.
+   *
+   * @param aObserver observer for all script processing.
+   */
+  nsresult AddObserver(nsIScriptLoaderObserver* aObserver) {
+    return mObservers.AppendObject(aObserver) ? NS_OK : NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  /**
+   * Remove an observer.
+   *
+   * @param aObserver observer to be removed
+   */
+  void RemoveObserver(nsIScriptLoaderObserver* aObserver) {
+    mObservers.RemoveObject(aObserver);
+  }
+
+  /**
+   * Process a script element. This will include both loading the
+   * source of the element if it is not inline and evaluating
+   * the script itself.
+   *
+   * If the script is an inline script that can be executed immediately
+   * (i.e. there are no other scripts pending) then ScriptAvailable
+   * and ScriptEvaluated will be called before the function returns.
+   *
+   * If true is returned the script could not be executed immediately.
+   * In this case ScriptAvailable is guaranteed to be called at a later
+   * point (as well as possibly ScriptEvaluated).
+   *
+   * @param aElement The element representing the script to be loaded and
+   *        evaluated.
+   */
+  bool ProcessScriptElement(nsIScriptElement* aElement);
+
+  /**
+   * Gets the currently executing script. This is useful if you want to
+   * generate a unique key based on the currently executing script.
+   */
+  nsIScriptElement* GetCurrentScript() { return mCurrentScript; }
+
+  nsIScriptElement* GetCurrentParserInsertedScript() {
+    return mCurrentParserInsertedScript;
+  }
+
+  /**
+   * Whether the loader is enabled or not.
+   * When disabled, processing of new script elements is disabled.
+   * Any call to ProcessScriptElement() will return false. Note that
+   * this DOES NOT disable currently loading or executing scripts.
+   */
+  bool GetEnabled() { return mEnabled; }
+
+  void SetEnabled(bool aEnabled) {
+    if (!mEnabled && aEnabled) {
+      ProcessPendingRequestsAsync();
+    }
+    mEnabled = aEnabled;
+  }
+
+  ModuleLoader* GetModuleLoader() { return mModuleLoader; }
+
+  void RegisterContentScriptModuleLoader(ModuleLoader* aLoader);
+  void RegisterShadowRealmModuleLoader(ModuleLoader* aLoader);
+
+  /**
+   *  Check whether to speculatively OMT parse scripts as soon as
+   *  they are fetched, even if not a parser blocking request.
+   *  Controlled by
+   *  dom.script_loader.external_scripts.speculative_omt_parse_enabled
+   */
+  bool SpeculativeOMTParsingEnabled() const {
+    return mSpeculativeOMTParsingEnabled;
+  }
+
+  /**
+   * Add/remove a blocker for parser-blocking scripts (and XSLT
+   * scripts). Blockers will stop such scripts from executing, but not from
+   * loading.
+   */
+  void AddParserBlockingScriptExecutionBlocker() {
+    ++mParserBlockingBlockerCount;
+  }
+
+  void RemoveParserBlockingScriptExecutionBlocker() {
+    if (!--mParserBlockingBlockerCount && ReadyToExecuteScripts()) {
+      ProcessPendingRequestsAsync();
+    }
+  }
+
+  /**
+   * Add/remove a blocker for execution of all scripts.  Blockers will stop
+   * scripts from executing, but not from loading.
+   */
+  void AddExecuteBlocker() { ++mBlockerCount; }
+
+  void RemoveExecuteBlocker() {
+    MOZ_ASSERT(mBlockerCount);
+    if (!--mBlockerCount) {
+      ProcessPendingRequestsAsync();
+    }
+  }
+
+  /**
+   * Convert the given buffer to a UTF-16 string.  If the buffer begins with a
+   * BOM, it is interpreted as that encoding; otherwise the first of |aChannel|,
+   * |aHintCharset|, or |aDocument| that provides a recognized encoding is used,
+   * or Windows-1252 if none of them do.
+   *
+   * Encoding errors in the buffer are converted to replacement characters, so
+   * allocation failure is the only way this function can fail.
+   *
+   * @param aChannel     Channel corresponding to the data. May be null.
+   * @param aData        The data to convert
+   * @param aLength      Length of the data
+   * @param aHintCharset Character set hint (e.g., from a charset attribute).
+   * @param aDocument    Document which the data is loaded for. May be null.
+   * @param aBufOut      [out] fresh char16_t array containing data converted to
+   *                     Unicode.
+   * @param aLengthOut   [out] Length of array returned in aBufOut in number
+   *                     of char16_t code units.
+   */
+  static nsresult ConvertToUTF16(nsIChannel* aChannel, const uint8_t* aData,
+                                 uint32_t aLength,
+                                 const nsAString& aHintCharset,
+                                 Document* aDocument,
+                                 UniquePtr<char16_t[], JS::FreePolicy>& aBufOut,
+                                 size_t& aLengthOut);
+
+  /**
+   * Convert the given buffer to a UTF-8 string.  If the buffer begins with a
+   * BOM, it is interpreted as that encoding; otherwise the first of |aChannel|,
+   * |aHintCharset|, or |aDocument| that provides a recognized encoding is used,
+   * or Windows-1252 if none of them do.
+   *
+   * Encoding errors in the buffer are converted to replacement characters, so
+   * allocation failure is the only way this function can fail.
+   *
+   * @param aChannel     Channel corresponding to the data. May be null.
+   * @param aData        The data to convert
+   * @param aLength      Length of the data
+   * @param aHintCharset Character set hint (e.g., from a charset attribute).
+   * @param aDocument    Document which the data is loaded for. May be null.
+   * @param aBufOut      [out] fresh Utf8Unit array containing data converted to
+   *                     Unicode.
+   * @param aLengthOut   [out] Length of array returned in aBufOut in UTF-8 code
+   *                     units (i.e. in bytes).
+   */
+  static nsresult ConvertToUTF8(nsIChannel* aChannel, const uint8_t* aData,
+                                uint32_t aLength, const nsAString& aHintCharset,
+                                Document* aDocument,
+                                UniquePtr<Utf8Unit[], JS::FreePolicy>& aBufOut,
+                                size_t& aLengthOut);
+
+  /**
+   * Handle the completion of a stream.  This is called by the
+   * ScriptLoadHandler object which observes the IncrementalStreamLoader
+   * loading the script. The streamed content is expected to be stored on the
+   * aRequest argument.
+   */
+  nsresult OnStreamComplete(nsIIncrementalStreamLoader* aLoader,
+                            ScriptLoadRequest* aRequest,
+                            nsresult aChannelStatus, nsresult aSRIStatus,
+                            SRICheckDataVerifier* aSRIDataVerifier);
+
+  /**
+   * Returns wether any request is queued, and not executed yet.
+   */
+  bool HasPendingRequests() const;
+
+  /**
+   * Returns wether there are any dynamic module import requests pending.
+   */
+  bool HasPendingDynamicImports() const;
+
+  /**
+   * Processes any pending requests that are ready for processing.
+   */
+  void ProcessPendingRequests();
+
+  /**
+   * Starts deferring deferred scripts and puts them in the mDeferredRequests
+   * queue instead.
+   */
+  void BeginDeferringScripts();
+
+  /**
+   * Notifies the script loader that parsing is done.  If aTerminated is true,
+   * this will drop any pending scripts that haven't run yet, otherwise it will
+   * do nothing.
+   */
+  void ParsingComplete(bool aTerminated);
+
+  /**
+   * Notifies the script loader that the checkpoint to begin execution of defer
+   * scripts has been reached. This is either the end of of the document parse
+   * or the end of loading of parser-inserted stylesheets, whatever happens
+   * last.
+   *
+   * Otherwise, it will stop deferring scripts and immediately processes the
+   * mDeferredRequests queue.
+   *
+   * WARNING: This function will synchronously execute content scripts, so be
+   * prepared that the world might change around you.
+   */
+  void DeferCheckpointReached();
+
+  /**
+   * Returns the number of pending scripts, deferred or not.
+   */
+  uint32_t HasPendingOrCurrentScripts() {
+    return mCurrentScript || mParserBlockingRequest;
+  }
+
+  /**
+   * Adds aURI to the preload list and starts loading it.
+   *
+   * @param aURI The URI of the external script.
+   * @param aCharset The charset parameter for the script.
+   * @param aType The type parameter for the script.
+   * @param aCrossOrigin The crossorigin attribute for the script.
+   *                     Void if not present.
+   * @param aFetchPriority
+   * <https://html.spec.whatwg.org/#the-script-element:attr-script-fetchpriority>.
+   * @param aIntegrity The expect hash url, if avail, of the request
+
+   * @param aScriptFromHead Whether or not the script was a child of head
+   */
+  virtual void PreloadURI(nsIURI* aURI, const nsAString& aCharset,
+                          const nsAString& aType, const nsAString& aCrossOrigin,
+                          const nsAString& aNonce,
+                          const nsAString& aFetchPriority,
+                          const nsAString& aIntegrity, bool aScriptFromHead,
+                          bool aAsync, bool aDefer, bool aLinkPreload,
+                          const ReferrerPolicy aReferrerPolicy,
+                          uint64_t aEarlyHintPreloaderId);
+
+  /**
+   * Process a request that was deferred so that the script could be compiled
+   * off thread.
+   */
+  nsresult ProcessOffThreadRequest(ScriptLoadRequest* aRequest);
+
+  bool AddPendingChildLoader(ScriptLoader* aChild) {
+    // XXX(Bug 1631371) Check if this should use a fallible operation as it
+    // pretended earlier. Else, change the return type to void.
+    mPendingChildLoaders.AppendElement(aChild);
+    return true;
+  }
+
+  mozilla::dom::DocGroup* GetDocGroup() const;
+
+  /**
+   * Register the fact that we saw the load event, and that we need to save the
+   * bytecode at the next loop cycle unless new scripts are waiting in the
+   * pipeline.
+   */
+  void LoadEventFired();
+
+  /**
+   * Destroy and prevent the ScriptLoader or the ScriptLoadRequests from owning
+   * any references to the JSScript or to the Request which might be used for
+   * caching the encoded bytecode.
+   */
+  void Destroy();
+
+  /*
+   * Get the currently active script. This is used as the initiating script when
+   * executing timeout handler scripts.
+   */
+  static JS::loader::LoadedScript* GetActiveScript(JSContext* aCx);
+
+  Document* GetDocument() const { return mDocument; }
+
+  nsIURI* GetBaseURI() const override;
+
+ private:
+  ~ScriptLoader();
+
+  already_AddRefed<ScriptLoadRequest> CreateLoadRequest(
+      ScriptKind aKind, nsIURI* aURI, nsIScriptElement* aElement,
+      nsIPrincipal* aTriggeringPrincipal, mozilla::CORSMode aCORSMode,
+      const nsAString& aNonce, RequestPriority aRequestPriority,
+      const SRIMetadata& aIntegrity, ReferrerPolicy aReferrerPolicy,
+      JS::loader::ParserMetadata aParserMetadata);
+
+  /**
+   * Unblocks the creator parser of the parser-blocking scripts.
+   */
+  void UnblockParser(ScriptLoadRequest* aParserBlockingRequest);
+
+  /**
+   * Asynchronously resumes the creator parser of the parser-blocking scripts.
+   */
+  void ContinueParserAsync(ScriptLoadRequest* aParserBlockingRequest);
+
+  bool ProcessExternalScript(nsIScriptElement* aElement, ScriptKind aScriptKind,
+                             nsIContent* aScriptContent);
+
+  bool ProcessInlineScript(nsIScriptElement* aElement, ScriptKind aScriptKind);
+
+  JS::loader::ScriptLoadRequest* LookupPreloadRequest(
+      nsIScriptElement* aElement, ScriptKind aScriptKind,
+      const SRIMetadata& aSRIMetadata);
+
+  void GetSRIMetadata(const nsAString& aIntegrityAttr,
+                      SRIMetadata* aMetadataOut);
+
+  /**
+   * Given a script element, get the referrer policy should be applied to load
+   * requests.
+   */
+  ReferrerPolicy GetReferrerPolicy(nsIScriptElement* aElement);
+
+  /**
+   * Helper function to check the content policy for a given request.
+   */
+  static nsresult CheckContentPolicy(Document* aDocument,
+                                     nsIScriptElement* aElement,
+                                     const nsAString& aNonce,
+                                     ScriptLoadRequest* aRequest);
+
+  /**
+   * Helper function to determine whether an about: page loads a chrome: URI.
+   * Please note that this function only returns true if:
+   *   * the about: page uses a ContentPrincipal with scheme about:
+   *   * the about: page is not linkable from content
+   *     (e.g. the function will return false for about:blank or about:srcdoc)
+   */
+  static bool IsAboutPageLoadingChromeURI(ScriptLoadRequest* aRequest,
+                                          Document* aDocument);
+
+  /**
+   * Start a load for aRequest's URI.
+   */
+  nsresult StartLoad(ScriptLoadRequest* aRequest,
+                     const Maybe<nsAutoString>& aCharsetForPreload);
+  /**
+   * Start a load for a classic script URI.
+   * Sets up the necessary security flags before calling StartLoadInternal.
+   */
+  nsresult StartClassicLoad(ScriptLoadRequest* aRequest,
+                            const Maybe<nsAutoString>& aCharsetForPreload);
+
+  static void PrepareCacheInfoChannel(nsIChannel* aChannel,
+                                      ScriptLoadRequest* aRequest);
+
+  static void PrepareRequestPriorityAndRequestDependencies(
+      nsIChannel* aChannel, ScriptLoadRequest* aRequest);
+
+  [[nodiscard]] static nsresult PrepareHttpRequestAndInitiatorType(
+      nsIChannel* aChannel, ScriptLoadRequest* aRequest,
+      const Maybe<nsAutoString>& aCharsetForPreload);
+
+  [[nodiscard]] nsresult PrepareIncrementalStreamLoader(
+      nsIIncrementalStreamLoader** aOutLoader, ScriptLoadRequest* aRequest);
+
+  /**
+   * Start a load for a script (module or classic) URI.
+   *
+   * aCharsetForPreload is only needed when this load is a preload (via
+   * ScriptLoader::PreloadURI), because ScriptLoadRequest doesn't
+   * have this information.
+   */
+  nsresult StartLoadInternal(ScriptLoadRequest* aRequest,
+                             nsSecurityFlags securityFlags,
+                             const Maybe<nsAutoString>& aCharsetForPreload);
+
+  /**
+   * Abort the current stream, and re-start with a new load request from scratch
+   * without requesting any alternate data. Returns NS_BINDING_RETARGETED on
+   * success, as this error code is used to abort the input stream.
+   */
+  nsresult RestartLoad(ScriptLoadRequest* aRequest);
+
+  void HandleLoadError(ScriptLoadRequest* aRequest, nsresult aResult);
+
+  /**
+   * Process any pending requests asynchronously (i.e. off an event) if there
+   * are any. Note that this is a no-op if there aren't any currently pending
+   * requests.
+   *
+   * This function is virtual to allow cross-library calls to SetEnabled()
+   */
+  virtual void ProcessPendingRequestsAsync();
+
+  /**
+   * If true, the loader is ready to execute parser-blocking scripts, and so are
+   * all its ancestors.  If the loader itself is ready but some ancestor is not,
+   * this function will add an execute blocker and ask the ancestor to remove it
+   * once it becomes ready.
+   */
+  bool ReadyToExecuteParserBlockingScripts();
+
+  /**
+   * Return whether just this loader is ready to execute parser-blocking
+   * scripts.
+   */
+  bool SelfReadyToExecuteParserBlockingScripts() {
+    return ReadyToExecuteScripts() && !mParserBlockingBlockerCount;
+  }
+
+  /**
+   * Return whether this loader is ready to execute scripts in general.
+   */
+  bool ReadyToExecuteScripts() { return mEnabled && !mBlockerCount; }
+
+  nsresult VerifySRI(ScriptLoadRequest* aRequest,
+                     nsIIncrementalStreamLoader* aLoader, nsresult aSRIStatus,
+                     SRICheckDataVerifier* aSRIDataVerifier) const;
+
+  nsresult SaveSRIHash(ScriptLoadRequest* aRequest,
+                       SRICheckDataVerifier* aSRIDataVerifier,
+                       uint32_t* sriLength) const;
+
+  void ReportErrorToConsole(ScriptLoadRequest* aRequest,
+                            nsresult aResult) const override;
+
+  void ReportWarningToConsole(
+      ScriptLoadRequest* aRequest, const char* aMessageName,
+      const nsTArray<nsString>& aParams = nsTArray<nsString>()) const override;
+
+  void ReportPreloadErrorsToConsole(ScriptLoadRequest* aRequest);
+
+  nsresult AttemptOffThreadScriptCompile(ScriptLoadRequest* aRequest,
+                                         bool* aCouldCompileOut);
+
+  nsresult CreateOffThreadTask(JSContext* aCx, ScriptLoadRequest* aRequest,
+                               JS::CompileOptions& aOptions,
+                               CompileOrDecodeTask** aCompileOrDecodeTask);
+
+  nsresult ProcessRequest(ScriptLoadRequest* aRequest);
+  nsresult CompileOffThreadOrProcessRequest(ScriptLoadRequest* aRequest);
+  void FireScriptAvailable(nsresult aResult, ScriptLoadRequest* aRequest);
+  // TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230)
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void FireScriptEvaluated(
+      nsresult aResult, ScriptLoadRequest* aRequest);
+
+  // Implements https://html.spec.whatwg.org/#execute-the-script-block
+  nsresult EvaluateScriptElement(ScriptLoadRequest* aRequest);
+
+  // Handles both bytecode and text source scripts; populates exec with a
+  // compiled script
+  nsresult CompileOrDecodeClassicScript(JSContext* aCx,
+                                        JSExecutionContext& aExec,
+                                        ScriptLoadRequest* aRequest);
+
+  static nsCString& BytecodeMimeTypeFor(ScriptLoadRequest* aRequest);
+
+  // Decide whether to encode bytecode for given script load request,
+  // and store the script into the request if necessary.
+  //
+  // This method must be called before executing the script.
+  void MaybePrepareForBytecodeEncodingBeforeExecute(
+      ScriptLoadRequest* aRequest, JS::Handle<JSScript*> aScript);
+
+  // Queue the script load request for bytecode encoding if we decided to
+  // encode, or cleanup the script load request fields otherwise.
+  //
+  // This method must be called after executing the script.
+  nsresult MaybePrepareForBytecodeEncodingAfterExecute(
+      ScriptLoadRequest* aRequest, nsresult aRv);
+
+  // Returns true if MaybePrepareForBytecodeEncodingAfterExecute is called
+  // for given script load request.
+  bool IsAlreadyHandledForBytecodeEncodingPreparation(
+      ScriptLoadRequest* aRequest);
+
+  void MaybePrepareModuleForBytecodeEncodingBeforeExecute(
+      JSContext* aCx, ModuleLoadRequest* aRequest) override;
+
+  nsresult MaybePrepareModuleForBytecodeEncodingAfterExecute(
+      ModuleLoadRequest* aRequest, nsresult aRv) override;
+
+  // Implements https://html.spec.whatwg.org/#run-a-classic-script
+  nsresult EvaluateScript(nsIGlobalObject* aGlobalObject,
+                          ScriptLoadRequest* aRequest);
+
+  /**
+   * Queue the current script load request to be saved, when the page
+   * initialization ends. The page initialization end is defined as being the
+   * time when the load event got received, and when no more scripts are waiting
+   * to be executed.
+   */
+  void RegisterForBytecodeEncoding(ScriptLoadRequest* aRequest);
+
+  /**
+   * Check if all conditions are met, i-e that the onLoad event fired and that
+   * no more script have to be processed.  If all conditions are met, queue an
+   * event to encode all the bytecode and save them on the cache.
+   */
+  void MaybeTriggerBytecodeEncoding() override;
+
+  /**
+   * Iterate over all script load request and save the bytecode of executed
+   * functions on the cache provided by the channel.
+   */
+  void EncodeBytecode();
+  void EncodeRequestBytecode(JSContext* aCx, ScriptLoadRequest* aRequest);
+
+  void GiveUpBytecodeEncoding();
+
+  already_AddRefed<nsIGlobalObject> GetGlobalForRequest(
+      ScriptLoadRequest* aRequest);
+
+  already_AddRefed<nsIScriptGlobalObject> GetScriptGlobalObject();
+
+  // Fill in CompileOptions, as well as produce the introducer script for
+  // subsequent calls to UpdateDebuggerMetadata
+  nsresult FillCompileOptionsForRequest(
+      JSContext* aCx, ScriptLoadRequest* aRequest, JS::CompileOptions* aOptions,
+      JS::MutableHandle<JSScript*> aIntroductionScript) override;
+
+  uint32_t NumberOfProcessors();
+  int32_t PhysicalSizeOfMemoryInGB();
+
+  nsresult PrepareLoadedRequest(ScriptLoadRequest* aRequest,
+                                nsIIncrementalStreamLoader* aLoader,
+                                nsresult aStatus);
+
+  void AddDeferRequest(ScriptLoadRequest* aRequest);
+  void AddAsyncRequest(ScriptLoadRequest* aRequest);
+  bool MaybeRemovedDeferRequests();
+
+  bool ShouldApplyDelazifyStrategy(ScriptLoadRequest* aRequest);
+  void ApplyDelazifyStrategy(JS::CompileOptions* aOptions);
+
+  bool ShouldCompileOffThread(ScriptLoadRequest* aRequest);
+
+  void MaybeMoveToLoadedList(ScriptLoadRequest* aRequest);
+
+  // Returns wether we should save the bytecode of this script after the
+  // execution of the script.
+  static bool ShouldCacheBytecode(ScriptLoadRequest* aRequest);
+
+  void RunScriptWhenSafe(ScriptLoadRequest* aRequest);
+
+  /**
+   * Cancel and remove all outstanding load requests, including waiting for any
+   * off thread compilations to finish.
+   */
+  void CancelAndClearScriptLoadRequests();
+
+  Document* mDocument;  // [WEAK]
+  nsCOMArray<nsIScriptLoaderObserver> mObservers;
+  ScriptLoadRequestList mNonAsyncExternalScriptInsertedRequests;
+  // mLoadingAsyncRequests holds async requests while they're loading; when they
+  // have been loaded they are moved to mLoadedAsyncRequests.
+  ScriptLoadRequestList mLoadingAsyncRequests;
+  // mLoadedAsyncRequests holds async script requests and dynamic module import
+  // requests, which are processed in the same way.
+  ScriptLoadRequestList mLoadedAsyncRequests;
+  ScriptLoadRequestList mDeferRequests;
+  ScriptLoadRequestList mXSLTRequests;
+  RefPtr<ScriptLoadRequest> mParserBlockingRequest;
+  ScriptLoadRequestList mOffThreadCompilingRequests;
+
+  // List of script load request that are holding a buffer which has to be saved
+  // on the cache.
+  ScriptLoadRequestList mBytecodeEncodingQueue;
+
+  // In mRequests, the additional information here is stored by the element.
+  struct PreloadInfo {
+    RefPtr<ScriptLoadRequest> mRequest;
+    nsString mCharset;
+  };
+
+  friend void ImplCycleCollectionUnlink(ScriptLoader::PreloadInfo& aField);
+  friend void ImplCycleCollectionTraverse(
+      nsCycleCollectionTraversalCallback& aCallback,
+      ScriptLoader::PreloadInfo& aField, const char* aName, uint32_t aFlags);
+
+  struct PreloadRequestComparator {
+    bool Equals(const PreloadInfo& aPi,
+                ScriptLoadRequest* const& aRequest) const {
+      return aRequest == aPi.mRequest;
+    }
+  };
+
+  struct PreloadURIComparator {
+    bool Equals(const PreloadInfo& aPi, nsIURI* const& aURI) const;
+  };
+
+  nsTArray<PreloadInfo> mPreloads;
+
+  nsCOMPtr<nsIScriptElement> mCurrentScript;
+  nsCOMPtr<nsIScriptElement> mCurrentParserInsertedScript;
+  nsTArray<RefPtr<ScriptLoader>> mPendingChildLoaders;
+  uint32_t mParserBlockingBlockerCount;
+  uint32_t mBlockerCount;
+  uint32_t mNumberOfProcessors;
+  uint32_t mTotalFullParseSize;
+  int32_t mPhysicalSizeOfMemory;
+  bool mEnabled;
+  bool mDeferEnabled;
+  bool mSpeculativeOMTParsingEnabled;
+  bool mDeferCheckpointReached;
+  bool mBlockingDOMContentLoaded;
+  bool mLoadEventFired;
+  bool mGiveUpEncoding;
+
+  TimeDuration mMainThreadParseTime;
+
+  nsCOMPtr<nsIConsoleReportCollector> mReporter;
+
+  // ShutdownObserver for off thread compilations
+  RefPtr<AsyncCompileShutdownObserver> mShutdownObserver;
+
+  RefPtr<ModuleLoader> mModuleLoader;
+  nsTArray<RefPtr<ModuleLoader>> mWebExtModuleLoaders;
+  nsTArray<RefPtr<ModuleLoader>> mShadowRealmModuleLoaders;
+
+  // Logging
+ public:
+  static LazyLogModule gCspPRLog;
+  static LazyLogModule gScriptLoaderLog;
+};
+
+class nsAutoScriptLoaderDisabler {
+ public:
+  explicit nsAutoScriptLoaderDisabler(Document* aDoc);
+
+  ~nsAutoScriptLoaderDisabler();
+
+  bool mWasEnabled;
+  RefPtr<ScriptLoader> mLoader;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_ScriptLoader_h
diff --git a/dom/script/ScriptSettings.cpp b/dom/script/ScriptSettings.cpp
new file mode 100644
index 0000000000..3b39538e51
--- /dev/null
+++ b/dom/script/ScriptSettings.cpp
@@ -0,0 +1,709 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/dom/ScriptSettings.h"
+
+#include <utility>
+#include "MainThreadUtils.h"
+#include "js/CharacterEncoding.h"
+#include "js/CompilationAndEvaluation.h"
+#include "js/Conversions.h"
+#include "js/ErrorReport.h"
+#include "js/Exception.h"
+#include "js/GCAPI.h"
+#include "js/PropertyAndElement.h"  // JS_GetProperty
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "js/Warnings.h"
+#include "js/Wrapper.h"
+#include "js/friend/ErrorMessages.h"
+#include "js/loader/LoadedScript.h"
+#include "js/loader/ScriptLoadRequest.h"
+#include "jsapi.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/BasePrincipal.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ThreadLocal.h"
+#include "mozilla/dom/AutoEntryScript.h"
+#include "mozilla/dom/BindingUtils.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/WorkerCommon.h"
+#include "nsContentUtils.h"
+#include "nsDebug.h"
+#include "nsGlobalWindowInner.h"
+#include "nsIGlobalObject.h"
+#include "nsINode.h"
+#include "nsIPrincipal.h"
+#include "nsISupports.h"
+#include "nsJSUtils.h"
+#include "nsPIDOMWindow.h"
+#include "nsString.h"
+#include "nscore.h"
+#include "xpcpublic.h"
+
+namespace mozilla {
+namespace dom {
+
+static MOZ_THREAD_LOCAL(ScriptSettingsStackEntry*) sScriptSettingsTLS;
+
+class ScriptSettingsStack {
+ public:
+  static ScriptSettingsStackEntry* Top() { return sScriptSettingsTLS.get(); }
+
+  static void Push(ScriptSettingsStackEntry* aEntry) {
+    MOZ_ASSERT(!aEntry->mOlder);
+    // Whenever JSAPI use is disabled, the next stack entry pushed must
+    // not be an AutoIncumbentScript.
+    MOZ_ASSERT_IF(!Top() || Top()->NoJSAPI(), !aEntry->IsIncumbentScript());
+    // Whenever the top entry is not an incumbent canidate, the next stack entry
+    // pushed must not be an AutoIncumbentScript.
+    MOZ_ASSERT_IF(Top() && !Top()->IsIncumbentCandidate(),
+                  !aEntry->IsIncumbentScript());
+
+    aEntry->mOlder = Top();
+    sScriptSettingsTLS.set(aEntry);
+  }
+
+  static void Pop(ScriptSettingsStackEntry* aEntry) {
+    MOZ_ASSERT(aEntry == Top());
+    sScriptSettingsTLS.set(aEntry->mOlder);
+  }
+
+  static nsIGlobalObject* IncumbentGlobal() {
+    ScriptSettingsStackEntry* entry = Top();
+    while (entry) {
+      if (entry->IsIncumbentCandidate()) {
+        return entry->mGlobalObject;
+      }
+      entry = entry->mOlder;
+    }
+    return nullptr;
+  }
+
+  static ScriptSettingsStackEntry* EntryPoint() {
+    ScriptSettingsStackEntry* entry = Top();
+    while (entry) {
+      if (entry->IsEntryCandidate()) {
+        return entry;
+      }
+      entry = entry->mOlder;
+    }
+    return nullptr;
+  }
+
+  static nsIGlobalObject* EntryGlobal() {
+    ScriptSettingsStackEntry* entry = EntryPoint();
+    if (!entry) {
+      return nullptr;
+    }
+    return entry->mGlobalObject;
+  }
+
+#ifdef DEBUG
+  static ScriptSettingsStackEntry* TopNonIncumbentScript() {
+    ScriptSettingsStackEntry* entry = Top();
+    while (entry) {
+      if (!entry->IsIncumbentScript()) {
+        return entry;
+      }
+      entry = entry->mOlder;
+    }
+    return nullptr;
+  }
+#endif  // DEBUG
+};
+
+void InitScriptSettings() {
+  bool success = sScriptSettingsTLS.init();
+  if (!success) {
+    MOZ_CRASH();
+  }
+
+  sScriptSettingsTLS.set(nullptr);
+}
+
+void DestroyScriptSettings() {
+  MOZ_ASSERT(sScriptSettingsTLS.get() == nullptr);
+}
+
+ScriptSettingsStackEntry::ScriptSettingsStackEntry(nsIGlobalObject* aGlobal,
+                                                   Type aType)
+    : mGlobalObject(aGlobal), mType(aType), mOlder(nullptr) {
+  MOZ_ASSERT_IF(IsIncumbentCandidate() && !NoJSAPI(), mGlobalObject);
+  MOZ_ASSERT(!mGlobalObject || mGlobalObject->HasJSGlobal(),
+             "Must have an actual JS global for the duration on the stack");
+  MOZ_ASSERT(
+      !mGlobalObject ||
+          JS_IsGlobalObject(mGlobalObject->GetGlobalJSObjectPreserveColor()),
+      "No outer windows allowed");
+}
+
+ScriptSettingsStackEntry::~ScriptSettingsStackEntry() {
+  // We must have an actual JS global for the entire time this is on the stack.
+  MOZ_ASSERT_IF(mGlobalObject, mGlobalObject->HasJSGlobal());
+}
+
+// If the entry or incumbent global ends up being something that the subject
+// principal doesn't subsume, we don't want to use it. This never happens on
+// the web, but can happen with asymmetric privilege relationships (i.e.
+// ExpandedPrincipal and System Principal).
+//
+// The most correct thing to use instead would be the topmost global on the
+// callstack whose principal is subsumed by the subject principal. But that's
+// hard to compute, so we just substitute the global of the current
+// compartment. In practice, this is fine.
+//
+// Note that in particular things like:
+//
+// |SpecialPowers.wrap(crossOriginWindow).eval(open())|
+//
+// trigger this case. Although both the entry global and the current global
+// have normal principals, the use of Gecko-specific System-Principaled JS
+// puts the code from two different origins on the callstack at once, which
+// doesn't happen normally on the web.
+static nsIGlobalObject* ClampToSubject(nsIGlobalObject* aGlobalOrNull) {
+  if (!aGlobalOrNull || !NS_IsMainThread()) {
+    return aGlobalOrNull;
+  }
+
+  nsIPrincipal* globalPrin = aGlobalOrNull->PrincipalOrNull();
+  NS_ENSURE_TRUE(globalPrin, GetCurrentGlobal());
+  if (!nsContentUtils::SubjectPrincipalOrSystemIfNativeCaller()
+           ->SubsumesConsideringDomain(globalPrin)) {
+    return GetCurrentGlobal();
+  }
+
+  return aGlobalOrNull;
+}
+
+nsIGlobalObject* GetEntryGlobal() {
+  return ClampToSubject(ScriptSettingsStack::EntryGlobal());
+}
+
+Document* GetEntryDocument() {
+  nsIGlobalObject* global = GetEntryGlobal();
+  nsCOMPtr<nsPIDOMWindowInner> entryWin = do_QueryInterface(global);
+
+  return entryWin ? entryWin->GetExtantDoc() : nullptr;
+}
+
+nsIGlobalObject* GetIncumbentGlobal() {
+  // We need the current JSContext in order to check the JS for
+  // scripted frames that may have appeared since anyone last
+  // manipulated the stack. If it's null, that means that there
+  // must be no entry global on the stack, and therefore no incumbent
+  // global either.
+  JSContext* cx = nsContentUtils::GetCurrentJSContext();
+  if (!cx) {
+    MOZ_ASSERT(ScriptSettingsStack::EntryGlobal() == nullptr);
+    return nullptr;
+  }
+
+  // See what the JS engine has to say. If we've got a scripted caller
+  // override in place, the JS engine will lie to us and pretend that
+  // there's nothing on the JS stack, which will cause us to check the
+  // incumbent script stack below.
+  if (JSObject* global = JS::GetScriptedCallerGlobal(cx)) {
+    return ClampToSubject(xpc::NativeGlobal(global));
+  }
+
+  // Ok, nothing from the JS engine. Let's use whatever's on the
+  // explicit stack.
+  return ClampToSubject(ScriptSettingsStack::IncumbentGlobal());
+}
+
+nsIGlobalObject* GetCurrentGlobal() {
+  JSContext* cx = nsContentUtils::GetCurrentJSContext();
+  if (!cx) {
+    return nullptr;
+  }
+
+  JSObject* global = JS::CurrentGlobalOrNull(cx);
+  if (!global) {
+    return nullptr;
+  }
+
+  return xpc::NativeGlobal(global);
+}
+
+nsIPrincipal* GetWebIDLCallerPrincipal() {
+  MOZ_ASSERT(NS_IsMainThread());
+  ScriptSettingsStackEntry* entry = ScriptSettingsStack::EntryPoint();
+
+  // If we have an entry point that is not NoJSAPI, we know it must be an
+  // AutoEntryScript.
+  if (!entry || entry->NoJSAPI()) {
+    return nullptr;
+  }
+  AutoEntryScript* aes = static_cast<AutoEntryScript*>(entry);
+
+  return aes->mWebIDLCallerPrincipal;
+}
+
+bool IsJSAPIActive() {
+  ScriptSettingsStackEntry* topEntry = ScriptSettingsStack::Top();
+  return topEntry && !topEntry->NoJSAPI();
+}
+
+namespace danger {
+JSContext* GetJSContext() { return CycleCollectedJSContext::Get()->Context(); }
+}  // namespace danger
+
+JS::RootingContext* RootingCx() {
+  return CycleCollectedJSContext::Get()->RootingCx();
+}
+
+AutoJSAPI::AutoJSAPI()
+    : ScriptSettingsStackEntry(nullptr, eJSAPI),
+      mCx(nullptr),
+      mIsMainThread(false)  // For lack of anything better
+{}
+
+AutoJSAPI::~AutoJSAPI() {
+  if (!mCx) {
+    // No need to do anything here: we never managed to Init, so can't have an
+    // exception on our (nonexistent) JSContext.  We also don't need to restore
+    // any state on it.  Finally, we never made it to pushing ourselves onto the
+    // ScriptSettingsStack, so shouldn't pop.
+    MOZ_ASSERT(ScriptSettingsStack::Top() != this);
+    return;
+  }
+
+  ReportException();
+
+  if (mOldWarningReporter.isSome()) {
+    JS::SetWarningReporter(cx(), mOldWarningReporter.value());
+  }
+
+  ScriptSettingsStack::Pop(this);
+}
+
+void WarningOnlyErrorReporter(JSContext* aCx, JSErrorReport* aRep);
+
+void AutoJSAPI::InitInternal(nsIGlobalObject* aGlobalObject, JSObject* aGlobal,
+                             JSContext* aCx, bool aIsMainThread) {
+  MOZ_ASSERT(aCx);
+  MOZ_ASSERT(aCx == danger::GetJSContext());
+  MOZ_ASSERT(aIsMainThread == NS_IsMainThread());
+  MOZ_ASSERT(bool(aGlobalObject) == bool(aGlobal));
+  MOZ_ASSERT_IF(aGlobalObject,
+                aGlobalObject->GetGlobalJSObjectPreserveColor() == aGlobal);
+#ifdef DEBUG
+  bool haveException = JS_IsExceptionPending(aCx);
+#endif  // DEBUG
+
+  mCx = aCx;
+  mIsMainThread = aIsMainThread;
+  if (aGlobal) {
+    JS::AssertObjectIsNotGray(aGlobal);
+  }
+  mAutoNullableRealm.emplace(mCx, aGlobal);
+  mGlobalObject = aGlobalObject;
+
+  ScriptSettingsStack::Push(this);
+
+  mOldWarningReporter.emplace(JS::GetWarningReporter(aCx));
+
+  JS::SetWarningReporter(aCx, WarningOnlyErrorReporter);
+
+#ifdef DEBUG
+  if (haveException) {
+    JS::Rooted<JS::Value> exn(aCx);
+    JS_GetPendingException(aCx, &exn);
+
+    JS_ClearPendingException(aCx);
+    if (exn.isObject()) {
+      JS::Rooted<JSObject*> exnObj(aCx, &exn.toObject());
+
+      // Make sure we can actually read things from it.  This UncheckedUwrap is
+      // safe because we're only getting data for a debug printf.  In
+      // particular, we do not expose this data to anyone, which is very
+      // important; otherwise it could be a cross-origin information leak.
+      exnObj = js::UncheckedUnwrap(exnObj);
+      JSAutoRealm ar(aCx, exnObj);
+
+      nsAutoJSString stack, filename, name, message;
+      int32_t line;
+
+      JS::Rooted<JS::Value> tmp(aCx);
+      if (!JS_GetProperty(aCx, exnObj, "filename", &tmp)) {
+        JS_ClearPendingException(aCx);
+      }
+      if (tmp.isUndefined()) {
+        if (!JS_GetProperty(aCx, exnObj, "fileName", &tmp)) {
+          JS_ClearPendingException(aCx);
+        }
+      }
+
+      if (!filename.init(aCx, tmp)) {
+        JS_ClearPendingException(aCx);
+      }
+
+      if (!JS_GetProperty(aCx, exnObj, "stack", &tmp) ||
+          !stack.init(aCx, tmp)) {
+        JS_ClearPendingException(aCx);
+      }
+
+      if (!JS_GetProperty(aCx, exnObj, "name", &tmp) || !name.init(aCx, tmp)) {
+        JS_ClearPendingException(aCx);
+      }
+
+      if (!JS_GetProperty(aCx, exnObj, "message", &tmp) ||
+          !message.init(aCx, tmp)) {
+        JS_ClearPendingException(aCx);
+      }
+
+      if (!JS_GetProperty(aCx, exnObj, "lineNumber", &tmp) ||
+          !JS::ToInt32(aCx, tmp, &line)) {
+        JS_ClearPendingException(aCx);
+        line = 0;
+      }
+
+      printf_stderr("PREEXISTING EXCEPTION OBJECT: '%s: %s'\n%s:%d\n%s\n",
+                    NS_ConvertUTF16toUTF8(name).get(),
+                    NS_ConvertUTF16toUTF8(message).get(),
+                    NS_ConvertUTF16toUTF8(filename).get(), line,
+                    NS_ConvertUTF16toUTF8(stack).get());
+    } else {
+      // It's a primitive... not much we can do other than stringify it.
+      nsAutoJSString exnStr;
+      if (!exnStr.init(aCx, exn)) {
+        JS_ClearPendingException(aCx);
+      }
+
+      printf_stderr("PREEXISTING EXCEPTION PRIMITIVE: %s\n",
+                    NS_ConvertUTF16toUTF8(exnStr).get());
+    }
+    MOZ_ASSERT(false, "We had an exception; we should not have");
+  }
+#endif  // DEBUG
+}
+
+AutoJSAPI::AutoJSAPI(nsIGlobalObject* aGlobalObject, bool aIsMainThread,
+                     Type aType)
+    : ScriptSettingsStackEntry(aGlobalObject, aType),
+      mIsMainThread(aIsMainThread) {
+  MOZ_ASSERT(aGlobalObject);
+  MOZ_ASSERT(aGlobalObject->HasJSGlobal(), "Must have a JS global");
+  MOZ_ASSERT(aIsMainThread == NS_IsMainThread());
+
+  InitInternal(aGlobalObject, aGlobalObject->GetGlobalJSObject(),
+               danger::GetJSContext(), aIsMainThread);
+}
+
+void AutoJSAPI::Init() {
+  MOZ_ASSERT(!mCx, "An AutoJSAPI should only be initialised once");
+
+  InitInternal(/* aGlobalObject */ nullptr, /* aGlobal */ nullptr,
+               danger::GetJSContext(), NS_IsMainThread());
+}
+
+bool AutoJSAPI::Init(nsIGlobalObject* aGlobalObject, JSContext* aCx) {
+  MOZ_ASSERT(!mCx, "An AutoJSAPI should only be initialised once");
+  MOZ_ASSERT(aCx);
+
+  if (NS_WARN_IF(!aGlobalObject)) {
+    return false;
+  }
+
+  JSObject* global = aGlobalObject->GetGlobalJSObject();
+  if (NS_WARN_IF(!global)) {
+    return false;
+  }
+
+  InitInternal(aGlobalObject, global, aCx, NS_IsMainThread());
+  return true;
+}
+
+bool AutoJSAPI::Init(nsIGlobalObject* aGlobalObject) {
+  return Init(aGlobalObject, danger::GetJSContext());
+}
+
+bool AutoJSAPI::Init(JSObject* aObject) {
+  MOZ_ASSERT(!js::IsCrossCompartmentWrapper(aObject));
+  return Init(xpc::NativeGlobal(aObject));
+}
+
+bool AutoJSAPI::Init(nsPIDOMWindowInner* aWindow, JSContext* aCx) {
+  return Init(nsGlobalWindowInner::Cast(aWindow), aCx);
+}
+
+bool AutoJSAPI::Init(nsPIDOMWindowInner* aWindow) {
+  return Init(nsGlobalWindowInner::Cast(aWindow));
+}
+
+bool AutoJSAPI::Init(nsGlobalWindowInner* aWindow, JSContext* aCx) {
+  return Init(static_cast<nsIGlobalObject*>(aWindow), aCx);
+}
+
+bool AutoJSAPI::Init(nsGlobalWindowInner* aWindow) {
+  return Init(static_cast<nsIGlobalObject*>(aWindow));
+}
+
+// Even with autoJSAPIOwnsErrorReporting, the JS engine still sends warning
+// reports to the JSErrorReporter as soon as they are generated. These go
+// directly to the console, so we can handle them easily here.
+//
+// Eventually, SpiderMonkey will have a special-purpose callback for warnings
+// only.
+void WarningOnlyErrorReporter(JSContext* aCx, JSErrorReport* aRep) {
+  MOZ_ASSERT(aRep->isWarning());
+  if (!NS_IsMainThread()) {
+    // Reporting a warning on workers is a bit complicated because we have to
+    // climb our parent chain until we get to the main thread.  So go ahead and
+    // just go through the worker or worklet ReportError codepath here.
+    //
+    // That said, it feels like we should be able to short-circuit things a bit
+    // here by posting an appropriate runnable to the main thread directly...
+    // Worth looking into sometime.
+    CycleCollectedJSContext* ccjscx = CycleCollectedJSContext::GetFor(aCx);
+    MOZ_ASSERT(ccjscx);
+
+    ccjscx->ReportError(aRep, JS::ConstUTF8CharsZ());
+    return;
+  }
+
+  RefPtr<xpc::ErrorReport> xpcReport = new xpc::ErrorReport();
+  nsGlobalWindowInner* win = xpc::CurrentWindowOrNull(aCx);
+  xpcReport->Init(aRep, nullptr, nsContentUtils::IsSystemCaller(aCx),
+                  win ? win->WindowID() : 0);
+  xpcReport->LogToConsole();
+}
+
+void AutoJSAPI::ReportException() {
+  if (!HasException()) {
+    return;
+  }
+
+  // AutoJSAPI uses a JSAutoNullableRealm, and may be in a null realm
+  // when the destructor is called. However, the JS engine requires us
+  // to be in a realm when we fetch the pending exception. In this case,
+  // we enter the privileged junk scope and don't dispatch any error events.
+  JS::Rooted<JSObject*> errorGlobal(cx(), JS::CurrentGlobalOrNull(cx()));
+  if (!errorGlobal) {
+    if (mIsMainThread) {
+      errorGlobal = xpc::PrivilegedJunkScope();
+    } else {
+      errorGlobal = GetCurrentThreadWorkerGlobal();
+      if (!errorGlobal) {
+        // We might be reporting an error in debugger code that ran before the
+        // worker's global was created. Use the debugger global instead.
+        errorGlobal = GetCurrentThreadWorkerDebuggerGlobal();
+        if (NS_WARN_IF(!errorGlobal)) {
+          // An exception may have been thrown on attempt to create a global
+          // and now there is no realm from which to fetch the exception.
+          // Give up.
+          ClearException();
+          return;
+        }
+      }
+    }
+  }
+  MOZ_ASSERT(JS_IsGlobalObject(errorGlobal));
+  JSAutoRealm ar(cx(), errorGlobal);
+  JS::ExceptionStack exnStack(cx());
+  JS::ErrorReportBuilder jsReport(cx());
+  if (StealExceptionAndStack(&exnStack) &&
+      jsReport.init(cx(), exnStack, JS::ErrorReportBuilder::WithSideEffects)) {
+    if (mIsMainThread) {
+      RefPtr<xpc::ErrorReport> xpcReport = new xpc::ErrorReport();
+
+      RefPtr<nsGlobalWindowInner> inner = xpc::WindowOrNull(errorGlobal);
+
+      // For WebExtension content script, `WindowOrNull` method will return
+      // null, whereas we would still like to flag the exception with the
+      // related WindowGlobal the content script executed against. So we only
+      // update the `innerWindowID` and not `inner` as we don't want to dispatch
+      // exceptions caused by the content script to the webpage.
+      uint64_t innerWindowID = 0;
+      if (inner) {
+        innerWindowID = inner->WindowID();
+      } else if (nsGlobalWindowInner* win = xpc::SandboxWindowOrNull(
+                     JS::GetNonCCWObjectGlobal(errorGlobal), cx())) {
+        innerWindowID = win->WindowID();
+      }
+
+      bool isChrome =
+          nsContentUtils::ObjectPrincipal(errorGlobal)->IsSystemPrincipal();
+      xpcReport->Init(jsReport.report(), jsReport.toStringResult().c_str(),
+                      isChrome, innerWindowID);
+      if (inner && jsReport.report()->errorNumber != JSMSG_OUT_OF_MEMORY) {
+        JS::RootingContext* rcx = JS::RootingContext::get(cx());
+        DispatchScriptErrorEvent(inner, rcx, xpcReport, exnStack.exception(),
+                                 exnStack.stack());
+      } else {
+        JS::Rooted<JSObject*> stack(cx());
+        JS::Rooted<JSObject*> stackGlobal(cx());
+        xpc::FindExceptionStackForConsoleReport(inner, exnStack.exception(),
+                                                exnStack.stack(), &stack,
+                                                &stackGlobal);
+        // This error is not associated with a specific window,
+        // so omit the exception value to mitigate potential leaks.
+        xpcReport->LogToConsoleWithStack(inner, JS::NothingHandleValue, stack,
+                                         stackGlobal);
+      }
+    } else {
+      // On a worker or worklet, we just use the error reporting mechanism and
+      // don't bother with xpc::ErrorReport.  This will ensure that all the
+      // right worker events (which are a lot more complicated than in the
+      // window case) get fired.
+      CycleCollectedJSContext* ccjscx = CycleCollectedJSContext::GetFor(cx());
+      MOZ_ASSERT(ccjscx);
+      // Before invoking ReportError, put the exception back on the context,
+      // because it may want to put it in its error events and has no other way
+      // to get hold of it.  After we invoke ReportError, clear the exception on
+      // cx(), just in case ReportError didn't.
+      JS::SetPendingExceptionStack(cx(), exnStack);
+      ccjscx->ReportError(jsReport.report(), jsReport.toStringResult());
+      ClearException();
+    }
+  } else {
+    NS_WARNING("OOMed while acquiring uncaught exception from JSAPI");
+    ClearException();
+  }
+}
+
+bool AutoJSAPI::PeekException(JS::MutableHandle<JS::Value> aVal) {
+  MOZ_ASSERT_IF(mIsMainThread, IsStackTop());
+  MOZ_ASSERT(HasException());
+  MOZ_ASSERT(js::GetContextRealm(cx()));
+  return JS_GetPendingException(cx(), aVal);
+}
+
+bool AutoJSAPI::StealException(JS::MutableHandle<JS::Value> aVal) {
+  JS::ExceptionStack exnStack(cx());
+  if (!StealExceptionAndStack(&exnStack)) {
+    return false;
+  }
+  aVal.set(exnStack.exception());
+  return true;
+}
+
+bool AutoJSAPI::StealExceptionAndStack(JS::ExceptionStack* aExnStack) {
+  MOZ_ASSERT_IF(mIsMainThread, IsStackTop());
+  MOZ_ASSERT(HasException());
+  MOZ_ASSERT(js::GetContextRealm(cx()));
+
+  return JS::StealPendingExceptionStack(cx(), aExnStack);
+}
+
+#ifdef DEBUG
+bool AutoJSAPI::IsStackTop() const {
+  return ScriptSettingsStack::TopNonIncumbentScript() == this;
+}
+#endif  // DEBUG
+
+AutoIncumbentScript::AutoIncumbentScript(nsIGlobalObject* aGlobalObject)
+    : ScriptSettingsStackEntry(aGlobalObject, eIncumbentScript),
+      mCallerOverride(nsContentUtils::GetCurrentJSContext()) {
+  ScriptSettingsStack::Push(this);
+}
+
+AutoIncumbentScript::~AutoIncumbentScript() { ScriptSettingsStack::Pop(this); }
+
+AutoNoJSAPI::AutoNoJSAPI(JSContext* aCx)
+    : ScriptSettingsStackEntry(nullptr, eNoJSAPI),
+      JSAutoNullableRealm(aCx, nullptr),
+      mCx(aCx) {
+  // Make sure we don't seem to have an incumbent global due to
+  // whatever script is running right now.
+  JS::HideScriptedCaller(aCx);
+
+  // Make sure the fallback GetIncumbentGlobal() behavior and
+  // GetEntryGlobal() both return null.
+  ScriptSettingsStack::Push(this);
+}
+
+AutoNoJSAPI::~AutoNoJSAPI() {
+  ScriptSettingsStack::Pop(this);
+  JS::UnhideScriptedCaller(mCx);
+}
+
+}  // namespace dom
+
+AutoJSContext::AutoJSContext() : mCx(nullptr) {
+  JS::AutoSuppressGCAnalysis nogc;
+  MOZ_ASSERT(!mCx, "mCx should not be initialized!");
+  MOZ_ASSERT(NS_IsMainThread());
+
+  if (dom::IsJSAPIActive()) {
+    mCx = dom::danger::GetJSContext();
+  } else {
+    mJSAPI.Init();
+    mCx = mJSAPI.cx();
+  }
+}
+
+AutoJSContext::operator JSContext*() const { return mCx; }
+
+AutoSafeJSContext::AutoSafeJSContext() {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  DebugOnly<bool> ok = Init(xpc::UnprivilegedJunkScope());
+  MOZ_ASSERT(ok,
+             "This is quite odd.  We should have crashed in the "
+             "xpc::NativeGlobal() call if xpc::UnprivilegedJunkScope() "
+             "returned null, and inited correctly otherwise!");
+}
+
+AutoSlowOperation::AutoSlowOperation() : mIsMainThread(NS_IsMainThread()) {
+  if (mIsMainThread) {
+    mScriptActivity.emplace(true);
+  }
+}
+
+void AutoSlowOperation::CheckForInterrupt() {
+  // For now we support only main thread!
+  if (mIsMainThread) {
+    // JS_CheckForInterrupt expects us to be in a realm, so we use a junk scope.
+    // In principle, it doesn't matter which one we use, since we aren't really
+    // running scripts here, and none of our interrupt callbacks can stop
+    // scripts in a junk scope anyway. In practice, though, the privileged junk
+    // scope is the same as the JSM global, and therefore always exists, while
+    // the unprivileged junk scope is created lazily, and may not exist until we
+    // try to use it. So we use the former for the sake of efficiency.
+    dom::AutoJSAPI jsapi;
+    MOZ_ALWAYS_TRUE(jsapi.Init(xpc::PrivilegedJunkScope()));
+    JS_CheckForInterrupt(jsapi.cx());
+  }
+}
+
+AutoAllowLegacyScriptExecution::AutoAllowLegacyScriptExecution() {
+#ifdef DEBUG
+  // no need to do that dance if we are off the main thread,
+  // because we only assert if we are on the main thread!
+  if (!NS_IsMainThread()) {
+    return;
+  }
+  sAutoAllowLegacyScriptExecution++;
+#endif
+}
+
+AutoAllowLegacyScriptExecution::~AutoAllowLegacyScriptExecution() {
+#ifdef DEBUG
+  // no need to do that dance if we are off the main thread,
+  // because we only assert if we are on the main thread!
+  if (!NS_IsMainThread()) {
+    return;
+  }
+  sAutoAllowLegacyScriptExecution--;
+  MOZ_ASSERT(sAutoAllowLegacyScriptExecution >= 0,
+             "how can the stack guard produce a value less than 0?");
+#endif
+}
+
+int AutoAllowLegacyScriptExecution::sAutoAllowLegacyScriptExecution = 0;
+
+/*static*/
+bool AutoAllowLegacyScriptExecution::IsAllowed() {
+  return sAutoAllowLegacyScriptExecution > 0;
+}
+
+}  // namespace mozilla
diff --git a/dom/script/ScriptSettings.h b/dom/script/ScriptSettings.h
new file mode 100644
index 0000000000..423da6c92a
--- /dev/null
+++ b/dom/script/ScriptSettings.h
@@ -0,0 +1,429 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Utilities for managing the script settings object stack defined in webapps */
+
+#ifndef mozilla_dom_ScriptSettings_h
+#define mozilla_dom_ScriptSettings_h
+
+#include "xpcpublic.h"
+
+#include "mozilla/dom/JSExecutionManager.h"
+#include "mozilla/Maybe.h"
+
+#include "jsapi.h"
+#include "js/Exception.h"
+#include "js/Warnings.h"  // JS::WarningReporter
+
+class JSObject;
+class nsIGlobalObject;
+class nsIPrincipal;
+class nsPIDOMWindowInner;
+class nsGlobalWindowInner;
+class nsIScriptContext;
+struct JSContext;
+
+namespace JS {
+class ExceptionStack;
+class Value;
+}  // namespace JS
+
+namespace mozilla {
+namespace dom {
+
+class Document;
+
+/*
+ * Per thread setup/teardown routines. Init and Destroy should be invoked
+ * once each, at startup and shutdown of the script runtime (respectively).
+ */
+void InitScriptSettings();
+void DestroyScriptSettings();
+
+// To implement a web-compatible browser, it is often necessary to obtain the
+// global object that is "associated" with the currently-running code. This
+// process is made more complicated by the fact that, historically, different
+// algorithms have operated with different definitions of the "associated"
+// global.
+//
+// HTML5 formalizes this into two concepts: the "incumbent global" and the
+// "entry global". The incumbent global corresponds to the global of the
+// current script being executed, whereas the entry global corresponds to the
+// global of the script where the current JS execution began.
+//
+// There is also a potentially-distinct third global that is determined by the
+// current compartment. This roughly corresponds with the notion of Realms in
+// ECMAScript.
+//
+// Suppose some event triggers an event listener in window |A|, which invokes a
+// scripted function in window |B|, which invokes the |window.location.href|
+// setter in window |C|. The entry global would be |A|, the incumbent global
+// would be |B|, and the current compartment would be that of |C|.
+//
+// In general, it's best to use to use the most-closely-associated global
+// unless the spec says to do otherwise. In 95% of the cases, the global of
+// the current compartment (GetCurrentGlobal()) is the right thing. For
+// example, WebIDL constructors (new C.XMLHttpRequest()) are initialized with
+// the global of the current compartment (i.e. |C|).
+//
+// The incumbent global is very similar, but differs in a few edge cases. For
+// example, if window |B| does |C.location.href = "..."|, the incumbent global
+// used for the navigation algorithm is B, because no script from |C| was ever
+// run.
+//
+// The entry global is used for various things like computing base URIs, mostly
+// for historical reasons.
+//
+// Note that all of these functions return bonafide global objects. This means
+// that, for Windows, they always return the inner.
+
+// Returns the global associated with the top-most Candidate Entry Point on
+// the Script Settings Stack. See the HTML spec. This may be null.
+nsIGlobalObject* GetEntryGlobal();
+
+// If the entry global is a window, returns its extant document. Otherwise,
+// returns null.
+Document* GetEntryDocument();
+
+// Returns the global associated with the top-most entry of the the Script
+// Settings Stack. See the HTML spec. This may be null.
+nsIGlobalObject* GetIncumbentGlobal();
+
+// Returns the global associated with the current compartment. This may be null.
+nsIGlobalObject* GetCurrentGlobal();
+
+// JS-implemented WebIDL presents an interesting situation with respect to the
+// subject principal. A regular C++-implemented API can simply examine the
+// compartment of the most-recently-executed script, and use that to infer the
+// responsible party. However, JS-implemented APIs are run with system
+// principal, and thus clobber the subject principal of the script that
+// invoked the API. So we have to do some extra work to keep track of this
+// information.
+//
+// We therefore implement the following behavior:
+// * Each Script Settings Object has an optional WebIDL Caller Principal field.
+//   This defaults to null.
+// * When we push an Entry Point in preparation to run a JS-implemented WebIDL
+//   callback, we grab the subject principal at the time of invocation, and
+//   store that as the WebIDL Caller Principal.
+// * When non-null, callers can query this principal from script via an API on
+//   Components.utils.
+nsIPrincipal* GetWebIDLCallerPrincipal();
+
+// Returns whether JSAPI is active right now.  If it is not, working with a
+// JSContext you grab from somewhere random is not OK and you should be doing
+// AutoJSAPI or AutoEntryScript to get yourself a properly set up JSContext.
+bool IsJSAPIActive();
+
+namespace danger {
+
+// Get the JSContext for this thread.  This is in the "danger" namespace because
+// we generally want people using AutoJSAPI instead, unless they really know
+// what they're doing.
+JSContext* GetJSContext();
+
+}  // namespace danger
+
+JS::RootingContext* RootingCx();
+
+class ScriptSettingsStack;
+class ScriptSettingsStackEntry {
+  friend class ScriptSettingsStack;
+
+ public:
+  ~ScriptSettingsStackEntry();
+
+  bool NoJSAPI() const { return mType == eNoJSAPI; }
+  bool IsEntryCandidate() const {
+    return mType == eEntryScript || mType == eNoJSAPI;
+  }
+  bool IsIncumbentCandidate() { return mType != eJSAPI; }
+  bool IsIncumbentScript() { return mType == eIncumbentScript; }
+
+ protected:
+  enum Type { eEntryScript, eIncumbentScript, eJSAPI, eNoJSAPI };
+
+  ScriptSettingsStackEntry(nsIGlobalObject* aGlobal, Type aEntryType);
+
+  nsCOMPtr<nsIGlobalObject> mGlobalObject;
+  Type mType;
+
+ private:
+  ScriptSettingsStackEntry* mOlder;
+};
+
+/*
+ * For any interaction with JSAPI, an AutoJSAPI (or one of its subclasses)
+ * must be on the stack.
+ *
+ * This base class should be instantiated as-is when the caller wants to use
+ * JSAPI but doesn't expect to run script. The caller must then call one of its
+ * Init functions before being able to access the JSContext through cx().
+ * Its current duties are as-follows (see individual Init comments for details):
+ *
+ * * Grabbing an appropriate JSContext, and, on the main thread, pushing it onto
+ *   the JSContext stack.
+ * * Entering an initial (possibly null) compartment, to ensure that the
+ *   previously entered compartment for that JSContext is not used by mistake.
+ * * Reporting any exceptions left on the JSRuntime, unless the caller steals
+ *   or silences them.
+ *
+ * Additionally, the following duties are planned, but not yet implemented:
+ *
+ * * De-poisoning the JSRuntime to allow manipulation of JSAPI. This requires
+ *   implementing the poisoning first.  For now, this de-poisoning
+ *   effectively corresponds to having a non-null cx on the stack.
+ *
+ * In situations where the consumer expects to run script, AutoEntryScript
+ * should be used, which does additional manipulation of the script settings
+ * stack. In bug 991758, we'll add hard invariants to SpiderMonkey, such that
+ * any attempt to run script without an AutoEntryScript on the stack will
+ * fail. This prevents system code from accidentally triggering script
+ * execution at inopportune moments via surreptitious getters and proxies.
+ */
+class MOZ_STACK_CLASS AutoJSAPI : protected ScriptSettingsStackEntry {
+ public:
+  // Trivial constructor. One of the Init functions must be called before
+  // accessing the JSContext through cx().
+  AutoJSAPI();
+
+  ~AutoJSAPI();
+
+  // This uses the SafeJSContext (or worker equivalent), and enters a null
+  // compartment, so that the consumer is forced to select a compartment to
+  // enter before manipulating objects.
+  //
+  // This variant will ensure that any errors reported by this AutoJSAPI as it
+  // comes off the stack will not fire error events or be associated with any
+  // particular web-visible global.
+  void Init();
+
+  // This uses the SafeJSContext (or worker equivalent), and enters the
+  // compartment of aGlobalObject.
+  // If aGlobalObject or its associated JS global are null then it returns
+  // false and use of cx() will cause an assertion.
+  //
+  // If aGlobalObject represents a web-visible global, errors reported by this
+  // AutoJSAPI as it comes off the stack will fire the relevant error events and
+  // show up in the corresponding web console.
+  //
+  // Successfully initializing the AutoJSAPI will ensure that it enters the
+  // Realm of aGlobalObject's JSObject and exposes that JSObject to active JS.
+  [[nodiscard]] bool Init(nsIGlobalObject* aGlobalObject);
+
+  // This is a helper that grabs the native global associated with aObject and
+  // invokes the above Init() with that. aObject must not be a cross-compartment
+  // wrapper: CCWs are not associated with a single global.
+  [[nodiscard]] bool Init(JSObject* aObject);
+
+  // Unsurprisingly, this uses aCx and enters the compartment of aGlobalObject.
+  // If aGlobalObject or its associated JS global are null then it returns
+  // false and use of cx() will cause an assertion.
+  // If aCx is null it will cause an assertion.
+  //
+  // If aGlobalObject represents a web-visible global, errors reported by this
+  // AutoJSAPI as it comes off the stack will fire the relevant error events and
+  // show up in the corresponding web console.
+  [[nodiscard]] bool Init(nsIGlobalObject* aGlobalObject, JSContext* aCx);
+
+  // Convenience functions to take an nsPIDOMWindowInner or nsGlobalWindowInner,
+  // when it is more easily available than an nsIGlobalObject.
+  [[nodiscard]] bool Init(nsPIDOMWindowInner* aWindow);
+  [[nodiscard]] bool Init(nsPIDOMWindowInner* aWindow, JSContext* aCx);
+
+  [[nodiscard]] bool Init(nsGlobalWindowInner* aWindow);
+  [[nodiscard]] bool Init(nsGlobalWindowInner* aWindow, JSContext* aCx);
+
+  JSContext* cx() const {
+    MOZ_ASSERT(mCx, "Must call Init before using an AutoJSAPI");
+    MOZ_ASSERT(IsStackTop());
+    return mCx;
+  }
+
+#ifdef DEBUG
+  bool IsStackTop() const;
+#endif
+
+  // If HasException, report it.  Otherwise, a no-op.
+  void ReportException();
+
+  bool HasException() const {
+    MOZ_ASSERT(IsStackTop());
+    return JS_IsExceptionPending(cx());
+  };
+
+  // Transfers ownership of the current exception from the JS engine to the
+  // caller. Callers must ensure that HasException() is true, and that cx()
+  // is in a non-null compartment.
+  //
+  // Note that this fails if and only if we OOM while wrapping the exception
+  // into the current compartment.
+  [[nodiscard]] bool StealException(JS::MutableHandle<JS::Value> aVal);
+
+  // As for StealException(), but uses the JS::ExceptionStack class to also
+  // include the exception's stack, represented by SavedFrames.
+  [[nodiscard]] bool StealExceptionAndStack(JS::ExceptionStack* aExnStack);
+
+  // Peek the current exception from the JS engine, without stealing it.
+  // Callers must ensure that HasException() is true, and that cx() is in a
+  // non-null compartment.
+  //
+  // Note that this fails if and only if we OOM while wrapping the exception
+  // into the current compartment.
+  [[nodiscard]] bool PeekException(JS::MutableHandle<JS::Value> aVal);
+
+  void ClearException() {
+    MOZ_ASSERT(IsStackTop());
+    JS_ClearPendingException(cx());
+  }
+
+ protected:
+  // Protected constructor for subclasses.  This constructor initialises the
+  // AutoJSAPI, so Init must NOT be called on subclasses that use this.
+  AutoJSAPI(nsIGlobalObject* aGlobalObject, bool aIsMainThread, Type aType);
+
+  mozilla::Maybe<JSAutoNullableRealm> mAutoNullableRealm;
+  JSContext* mCx;
+
+  // Whether we're mainthread or not; set when we're initialized.
+  bool mIsMainThread;
+  Maybe<JS::WarningReporter> mOldWarningReporter;
+
+ private:
+  void InitInternal(nsIGlobalObject* aGlobalObject, JSObject* aGlobal,
+                    JSContext* aCx, bool aIsMainThread);
+
+  AutoJSAPI(const AutoJSAPI&) = delete;
+  AutoJSAPI& operator=(const AutoJSAPI&) = delete;
+};
+
+/*
+ * A class that can be used to force a particular incumbent script on the stack.
+ */
+class AutoIncumbentScript : protected ScriptSettingsStackEntry {
+ public:
+  explicit AutoIncumbentScript(nsIGlobalObject* aGlobalObject);
+  ~AutoIncumbentScript();
+
+ private:
+  JS::AutoHideScriptedCaller mCallerOverride;
+};
+
+/*
+ * A class to put the JS engine in an unusable state. The subject principal
+ * will become System, the information on the script settings stack is
+ * rendered inaccessible, and JSAPI may not be manipulated until the class is
+ * either popped or an AutoJSAPI instance is subsequently pushed.
+ *
+ * This class may not be instantiated if an exception is pending.
+ */
+class AutoNoJSAPI : protected ScriptSettingsStackEntry,
+                    protected JSAutoNullableRealm {
+ public:
+  AutoNoJSAPI() : AutoNoJSAPI(danger::GetJSContext()) {}
+  ~AutoNoJSAPI();
+
+ private:
+  // Helper constructor to avoid doing GetJSContext() multiple times
+  // during construction.
+  explicit AutoNoJSAPI(JSContext* aCx);
+
+  // Stashed JSContext* so we don't need to GetJSContext in our destructor.
+  // It's probably safe to hold on to this, in the sense that the world should
+  // not get torn down while we're on the stack, and if it's not, we'd need to
+  // fix JSAutoNullableRealm to not hold on to a JSContext either, or
+  // something.
+  JSContext* mCx;
+
+  AutoYieldJSThreadExecution mExecutionYield;
+};
+
+}  // namespace dom
+
+/**
+ * Use AutoJSContext when you need a JS context on the stack but don't have one
+ * passed as a parameter. AutoJSContext will take care of finding the most
+ * appropriate JS context and release it when leaving the stack.
+ */
+class MOZ_RAII AutoJSContext {
+ public:
+  explicit AutoJSContext();
+  operator JSContext*() const;
+
+ protected:
+  JSContext* mCx;
+  dom::AutoJSAPI mJSAPI;
+};
+
+/**
+ * AutoSafeJSContext is similar to AutoJSContext but will only return the safe
+ * JS context. That means it will never call
+ * nsContentUtils::GetCurrentJSContext().
+ *
+ * Note - This is deprecated. Please use AutoJSAPI instead.
+ */
+class MOZ_RAII AutoSafeJSContext : public dom::AutoJSAPI {
+ public:
+  explicit AutoSafeJSContext();
+  operator JSContext*() const { return cx(); }
+
+ private:
+};
+
+/**
+ * Use AutoSlowOperation when native side calls many JS callbacks in a row
+ * and slow script dialog should be activated if too much time is spent going
+ * through those callbacks.
+ * AutoSlowOperation puts an AutoScriptActivity on the stack so that we don't
+ * continue to reset the watchdog. CheckForInterrupt can then be used to check
+ * whether JS execution should be interrupted.
+ * This class (including CheckForInterrupt) is a no-op when used off the main
+ * thread.
+ */
+class MOZ_RAII AutoSlowOperation {
+ public:
+  explicit AutoSlowOperation();
+  void CheckForInterrupt();
+
+ private:
+  bool mIsMainThread;
+  Maybe<xpc::AutoScriptActivity> mScriptActivity;
+};
+
+/**
+ * A class to disable interrupt callback temporary.
+ */
+class MOZ_RAII AutoDisableJSInterruptCallback {
+ public:
+  explicit AutoDisableJSInterruptCallback(JSContext* aCx)
+      : mCx(aCx), mOld(JS_DisableInterruptCallback(aCx)) {}
+
+  ~AutoDisableJSInterruptCallback() { JS_ResetInterruptCallback(mCx, mOld); }
+
+ private:
+  JSContext* mCx;
+  bool mOld;
+};
+
+/**
+ * A helper class which allows to allow-list legacy callers executing script
+ * in the AutoEntryScript constructor. The goal is to remove these exceptions
+ * one by one. Do not add a new one without review from a DOM peer.
+ */
+class MOZ_RAII AutoAllowLegacyScriptExecution {
+ public:
+  AutoAllowLegacyScriptExecution();
+  ~AutoAllowLegacyScriptExecution();
+
+  static bool IsAllowed();
+
+ private:
+  static int sAutoAllowLegacyScriptExecution;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_dom_ScriptSettings_h
diff --git a/dom/script/ScriptTrace.h b/dom/script/ScriptTrace.h
new file mode 100644
index 0000000000..29562d4f18
--- /dev/null
+++ b/dom/script/ScriptTrace.h
@@ -0,0 +1,57 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ScriptTrace_h
+#define mozilla_dom_ScriptTrace_h
+
+#include "mozilla/AsyncEventDispatcher.h"
+#include "mozilla/StaticPrefs_dom.h"
+
+// This macro is used to wrap a tracing mechanism which is scheduling events
+// which are then used by the JavaScript code of test cases to track the code
+// path to verify the optimizations are working as expected.
+#define TRACE_FOR_TEST(elem, str)                  \
+  PR_BEGIN_MACRO                                   \
+  nsresult rv = NS_OK;                             \
+  rv = mozilla::dom::script::TestingDispatchEvent( \
+      elem, NS_LITERAL_STRING_FROM_CSTRING(str));  \
+  NS_ENSURE_SUCCESS(rv, rv);                       \
+  PR_END_MACRO
+
+#define TRACE_FOR_TEST_BOOL(elem, str)             \
+  PR_BEGIN_MACRO                                   \
+  nsresult rv = NS_OK;                             \
+  rv = mozilla::dom::script::TestingDispatchEvent( \
+      elem, NS_LITERAL_STRING_FROM_CSTRING(str));  \
+  NS_ENSURE_SUCCESS(rv, false);                    \
+  PR_END_MACRO
+
+#define TRACE_FOR_TEST_NONE(elem, str)            \
+  PR_BEGIN_MACRO                                  \
+  mozilla::dom::script::TestingDispatchEvent(     \
+      elem, NS_LITERAL_STRING_FROM_CSTRING(str)); \
+  PR_END_MACRO
+
+namespace mozilla::dom::script {
+
+static nsresult TestingDispatchEvent(nsIScriptElement* aScriptElement,
+                                     const nsAString& aEventType) {
+  if (!StaticPrefs::dom_expose_test_interfaces()) {
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsINode> target(do_QueryInterface(aScriptElement));
+  if (!target) {
+    return NS_OK;
+  }
+
+  RefPtr<AsyncEventDispatcher> dispatcher = new AsyncEventDispatcher(
+      target, aEventType, CanBubble::eYes, ChromeOnlyDispatch::eNo);
+  return dispatcher->PostDOMEvent();
+}
+}  // namespace mozilla::dom::script
+
+#endif  // mozilla_dom_ScriptTrace_h
diff --git a/dom/script/ShadowRealmGlobalScope.cpp b/dom/script/ShadowRealmGlobalScope.cpp
new file mode 100644
index 0000000000..e455ccca49
--- /dev/null
+++ b/dom/script/ShadowRealmGlobalScope.cpp
@@ -0,0 +1,126 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsGlobalWindowInner.h"
+#include "nsIGlobalObject.h"
+#include "xpcpublic.h"
+#include "js/TypeDecls.h"
+
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/ModuleLoader.h"
+#include "mozilla/dom/ShadowRealmGlobalScope.h"
+#include "mozilla/dom/ShadowRealmGlobalScopeBinding.h"
+
+#include "js/loader/ModuleLoaderBase.h"
+
+using namespace JS::loader;
+
+namespace mozilla::dom {
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(ShadowRealmGlobalScope, mModuleLoader,
+                                      mCreatingGlobal)
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ShadowRealmGlobalScope)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ShadowRealmGlobalScope)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ShadowRealmGlobalScope)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+  NS_INTERFACE_MAP_ENTRY(nsIGlobalObject)
+  NS_INTERFACE_MAP_ENTRY(ShadowRealmGlobalScope)
+NS_INTERFACE_MAP_END
+
+JSObject* NewShadowRealmGlobal(JSContext* aCx, JS::RealmOptions& aOptions,
+                               JSPrincipals* aPrincipals,
+                               JS::Handle<JSObject*> aGlobalObj) {
+  JS::Rooted<JSObject*> reflector(aCx);
+  {
+    RefPtr<ShadowRealmGlobalScope> scope;
+    GlobalObject global(aCx, aGlobalObj);
+
+    nsCOMPtr<nsIGlobalObject> nsGlobal =
+        do_QueryInterface(global.GetAsSupports());
+
+    MOZ_ASSERT(nsGlobal);
+
+    scope = new ShadowRealmGlobalScope(nsGlobal);
+    ShadowRealmGlobalScope_Binding::Wrap(aCx, scope, scope, aOptions,
+                                         aPrincipals, &reflector);
+  }
+
+  return reflector;
+}
+
+static nsIGlobalObject* FindEnclosingNonShadowRealmGlobal(
+    ShadowRealmGlobalScope* scope) {
+  nsCOMPtr<nsIGlobalObject> global = scope->GetCreatingGlobal();
+
+  do {
+    nsCOMPtr<ShadowRealmGlobalScope> shadowRealmGlobalScope =
+        do_QueryInterface(global);
+    if (!shadowRealmGlobalScope) {
+      break;
+    }
+
+    // Our global was a ShadowRealmGlobal; that's a problem, as we can't find a
+    // window or worker global associated with a ShadowRealmGlobal... so we
+    // continue following the chain.
+    //
+    // This will happen if you have nested ShadowRealms.
+    global = shadowRealmGlobalScope->GetCreatingGlobal();
+  } while (true);
+
+  return global;
+}
+
+ModuleLoaderBase* ShadowRealmGlobalScope::GetModuleLoader(JSContext* aCx) {
+  if (mModuleLoader) {
+    return mModuleLoader;
+  }
+
+  // Note: if this fails, we don't need to report an exception, as one will be
+  // reported by ModuleLoaderBase::GetCurrentModuleLoader.
+
+  // Don't bother asking the ShadowRealmGlobal itself for a host object to get a
+  // module loader from, instead, delegate to the enclosing global of the shadow
+  // realm
+  nsCOMPtr<nsIGlobalObject> global = FindEnclosingNonShadowRealmGlobal(this);
+  MOZ_RELEASE_ASSERT(global);
+
+  JSObject* object = global->GetGlobalJSObject();
+  MOZ_ASSERT(object);
+
+  // Currently Workers will never get here, because dynamic import is disabled
+  // in Worker context, and so importValue will throw before we get here.
+  //
+  // See  https://bugzilla.mozilla.org/show_bug.cgi?id=1247687 and
+  //      https://bugzilla.mozilla.org/show_bug.cgi?id=1772162
+  nsGlobalWindowInner* window = xpc::WindowGlobalOrNull(object);
+  if (!window) {
+    return nullptr;
+  }
+
+  RefPtr<Document> doc = window->GetExtantDoc();
+  if (!doc) {
+    return nullptr;
+  }
+
+  ScriptLoader* scriptLoader = doc->ScriptLoader();
+
+  mModuleLoader = new ModuleLoader(scriptLoader, this, ModuleLoader::Normal);
+
+  // Register the shadow realm module loader for tracing and ownership.
+  scriptLoader->RegisterShadowRealmModuleLoader(
+      static_cast<ModuleLoader*>(mModuleLoader.get()));
+
+  return mModuleLoader;
+}
+
+bool IsShadowRealmGlobal(JSObject* aObject) {
+  return IS_INSTANCE_OF(ShadowRealmGlobalScope, aObject);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/script/ShadowRealmGlobalScope.h b/dom/script/ShadowRealmGlobalScope.h
new file mode 100644
index 0000000000..cf7b2a3bd3
--- /dev/null
+++ b/dom/script/ShadowRealmGlobalScope.h
@@ -0,0 +1,94 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ShadowRealmGlobalScope_h
+#define mozilla_dom_ShadowRealmGlobalScope_h
+
+#include "js/TypeDecls.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/OriginTrials.h"
+#include "mozilla/SchedulerGroup.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "nsContentUtils.h"
+#include "nsIGlobalObject.h"
+#include "nsWrapperCache.h"
+
+#include "js/loader/ModuleLoaderBase.h"
+
+namespace mozilla::dom {
+
+#define SHADOWREALMGLOBALSCOPE_IID                   \
+  { /* 1b0a59dd-c1cb-429a-bb90-cea17994dba2 */       \
+    0x1b0a59dd, 0xc1cb, 0x429a, {                    \
+      0xbb, 0x90, 0xce, 0xa1, 0x79, 0x94, 0xdb, 0xa2 \
+    }                                                \
+  }
+
+// Required for providing the wrapper, as this is the global used inside a Gecko
+// backed ShadowRealm, but also required to power module resolution.
+class ShadowRealmGlobalScope final : public nsIGlobalObject,
+                                     public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(ShadowRealmGlobalScope)
+
+  NS_DECLARE_STATIC_IID_ACCESSOR(SHADOWREALMGLOBALSCOPE_IID)
+
+  explicit ShadowRealmGlobalScope(nsIGlobalObject* aCreatingGlobal)
+      : mCreatingGlobal(aCreatingGlobal){};
+
+  nsIGlobalObject* GetCreatingGlobal() const { return mCreatingGlobal; }
+  OriginTrials Trials() const override { return {}; }
+
+  JSObject* GetGlobalJSObject() override { return GetWrapper(); }
+  JSObject* GetGlobalJSObjectPreserveColor() const override {
+    return GetWrapperPreserveColor();
+  }
+
+  virtual JSObject* WrapObject(JSContext* aCx,
+                               JS::Handle<JSObject*> aGivenProto) override {
+    MOZ_CRASH("Shouldn't be here");
+    return nullptr;
+  }
+
+  JS::loader::ModuleLoaderBase* GetModuleLoader(JSContext* aCx) override;
+
+  bool ShouldResistFingerprinting(RFPTarget aTarget) const override {
+    return nsContentUtils::ShouldResistFingerprinting(
+        "Presently we don't have enough context to make an informed decision"
+        "on JS Sandboxes. See 1782853",
+        aTarget);
+  }
+
+  nsISerialEventTarget* SerialEventTarget() const final {
+    return mozilla::GetMainThreadSerialEventTarget();
+  }
+  nsresult Dispatch(already_AddRefed<nsIRunnable>&& aRunnable) const final {
+    return mozilla::SchedulerGroup::Dispatch(std::move(aRunnable));
+  }
+
+ private:
+  virtual ~ShadowRealmGlobalScope() = default;
+
+  RefPtr<JS::loader::ModuleLoaderBase> mModuleLoader;
+
+  // The global which created this ShadowRealm
+  nsCOMPtr<nsIGlobalObject> mCreatingGlobal;
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(ShadowRealmGlobalScope,
+                              SHADOWREALMGLOBALSCOPE_IID)
+
+JSObject* NewShadowRealmGlobal(JSContext* aCx, JS::RealmOptions& aOptions,
+                               JSPrincipals* aPrincipals,
+                               JS::Handle<JSObject*> aGlobalObj);
+
+bool IsShadowRealmGlobal(JSObject* aObject);
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/script/moz.build b/dom/script/moz.build
new file mode 100644
index 0000000000..17fa695342
--- /dev/null
+++ b/dom/script/moz.build
@@ -0,0 +1,54 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+with Files("**"):
+    BUG_COMPONENT = ("Core", "DOM: Core & HTML")
+
+XPIDL_SOURCES += [
+    "nsIScriptLoaderObserver.idl",
+]
+
+XPIDL_MODULE = "dom"
+
+EXPORTS += [
+    "nsIScriptElement.h",
+]
+
+EXPORTS.mozilla.dom += [
+    "AutoEntryScript.h",
+    "ModuleLoader.h",
+    "ScriptCompression.h",
+    "ScriptDecoding.h",
+    "ScriptElement.h",
+    "ScriptLoadContext.h",
+    "ScriptLoader.h",
+    "ScriptLoadHandler.h",
+    "ScriptSettings.h",
+    "ScriptTrace.h",
+    "ShadowRealmGlobalScope.h",
+]
+
+UNIFIED_SOURCES += [
+    "AutoEntryScript.cpp",
+    "ModuleLoader.cpp",
+    "nsIScriptElement.cpp",
+    "ScriptCompression.cpp",
+    "ScriptElement.cpp",
+    "ScriptLoadContext.cpp",
+    "ScriptLoader.cpp",
+    "ScriptLoadHandler.cpp",
+    "ScriptSettings.cpp",
+    "ShadowRealmGlobalScope.cpp",
+]
+
+LOCAL_INCLUDES += [
+    "/dom/base",
+    "/js/loader",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")
+
+FINAL_LIBRARY = "xul"
diff --git a/dom/script/nsIScriptElement.cpp b/dom/script/nsIScriptElement.cpp
new file mode 100644
index 0000000000..7fda6f1fd4
--- /dev/null
+++ b/dom/script/nsIScriptElement.cpp
@@ -0,0 +1,102 @@
+
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsIScriptElement.h"
+
+#include "js/loader/ScriptKind.h"
+#include "mozilla/dom/Document.h"
+#include "mozilla/dom/ReferrerPolicyBinding.h"
+#include "nsIParser.h"
+#include "nsIWeakReference.h"
+
+using JS::loader::ScriptKind;
+
+bool nsIScriptElement::IsClassicNonAsyncDefer() {
+  return mKind == ScriptKind::eClassic && !mAsync && !mDefer;
+}
+
+void nsIScriptElement::SetCreatorParser(nsIParser* aParser) {
+  mCreatorParser = do_GetWeakReference(aParser);
+}
+
+void nsIScriptElement::UnblockParser() {
+  if (!IsClassicNonAsyncDefer()) {
+    MOZ_ASSERT_UNREACHABLE(
+        "Tried to unblock parser for a script type that cannot block "
+        "the parser.");
+    return;
+  }
+  nsCOMPtr<nsIParser> parser = do_QueryReferent(mCreatorParser);
+  if (parser) {
+    parser->UnblockParser();
+  }
+}
+
+void nsIScriptElement::ContinueParserAsync() {
+  if (!IsClassicNonAsyncDefer()) {
+    MOZ_ASSERT_UNREACHABLE(
+        "Tried to continue after a script type that cannot block the parser.");
+    return;
+  }
+  nsCOMPtr<nsIParser> parser = do_QueryReferent(mCreatorParser);
+  if (parser) {
+    parser->ContinueInterruptedParsingAsync();
+  }
+}
+
+void nsIScriptElement::BeginEvaluating() {
+  if (!IsClassicNonAsyncDefer()) {
+    return;
+  }
+  nsCOMPtr<nsIParser> parser = do_QueryReferent(mCreatorParser);
+  if (parser) {
+    parser->IncrementScriptNestingLevel();
+  }
+}
+
+void nsIScriptElement::EndEvaluating() {
+  if (!IsClassicNonAsyncDefer()) {
+    return;
+  }
+  nsCOMPtr<nsIParser> parser = do_QueryReferent(mCreatorParser);
+  if (parser) {
+    parser->DecrementScriptNestingLevel();
+  }
+}
+
+already_AddRefed<nsIParser> nsIScriptElement::GetCreatorParser() {
+  nsCOMPtr<nsIParser> parser = do_QueryReferent(mCreatorParser);
+  return parser.forget();
+}
+
+mozilla::dom::ReferrerPolicy nsIScriptElement::GetReferrerPolicy() {
+  return mozilla::dom::ReferrerPolicy::_empty;
+}
+
+void nsIScriptElement::DetermineKindFromType(
+    const mozilla::dom::Document* aOwnerDoc) {
+  MOZ_ASSERT((mKind != ScriptKind::eModule) &&
+             (mKind != ScriptKind::eImportMap) && !mAsync && !mDefer &&
+             !mExternal);
+
+  nsAutoString type;
+  GetScriptType(type);
+
+  if (!type.IsEmpty()) {
+    if (type.LowerCaseEqualsASCII("module")) {
+      mKind = ScriptKind::eModule;
+    }
+
+    // https://html.spec.whatwg.org/multipage/scripting.html#prepare-the-script-element
+    // Step 11. Otherwise, if the script block's type string is an ASCII
+    // case-insensitive match for the string "importmap", then set el's type to
+    // "importmap".
+    if (type.LowerCaseEqualsASCII("importmap")) {
+      mKind = ScriptKind::eImportMap;
+    }
+  }
+}
diff --git a/dom/script/nsIScriptElement.h b/dom/script/nsIScriptElement.h
new file mode 100644
index 0000000000..8601dbd182
--- /dev/null
+++ b/dom/script/nsIScriptElement.h
@@ -0,0 +1,375 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsIScriptElement_h___
+#define nsIScriptElement_h___
+
+#include "js/ColumnNumber.h"  // JS::ColumnNumberOneOrigin
+#include "js/loader/ScriptKind.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/CORSMode.h"
+#include "mozilla/dom/FromParser.h"
+#include "nsCOMPtr.h"
+#include "nsID.h"
+#include "nsIScriptLoaderObserver.h"
+#include "nsIWeakReferenceUtils.h"
+#include "nsStringFwd.h"
+#include "nscore.h"
+
+// XXX Avoid including this here by moving function bodies to the cpp file
+#include "nsIPrincipal.h"
+
+class nsIContent;
+class nsIParser;
+class nsIPrincipal;
+class nsIURI;
+
+namespace mozilla::dom {
+class Document;
+enum class FetchPriority : uint8_t;
+enum class ReferrerPolicy : uint8_t;
+}  // namespace mozilla::dom
+
+// Must be kept in sync with xpcom/rust/xpcom/src/interfaces/nonidl.rs
+#define NS_ISCRIPTELEMENT_IID                        \
+  {                                                  \
+    0xe60fca9b, 0x1b96, 0x4e4e, {                    \
+      0xa9, 0xb4, 0xdc, 0x98, 0x4f, 0x88, 0x3f, 0x9c \
+    }                                                \
+  }
+
+/**
+ * Internal interface implemented by script elements
+ */
+class nsIScriptElement : public nsIScriptLoaderObserver {
+ public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_ISCRIPTELEMENT_IID)
+
+  explicit nsIScriptElement(mozilla::dom::FromParser aFromParser)
+      : mLineNumber(1),
+        mColumnNumber(1),
+        mAlreadyStarted(false),
+        mMalformed(false),
+        mDoneAddingChildren(aFromParser == mozilla::dom::NOT_FROM_PARSER ||
+                            aFromParser == mozilla::dom::FROM_PARSER_FRAGMENT),
+        mForceAsync(aFromParser == mozilla::dom::NOT_FROM_PARSER ||
+                    aFromParser == mozilla::dom::FROM_PARSER_FRAGMENT),
+        mFrozen(false),
+        mDefer(false),
+        mAsync(false),
+        mExternal(false),
+        mKind(JS::loader::ScriptKind::eClassic),
+        mParserCreated(aFromParser == mozilla::dom::FROM_PARSER_FRAGMENT
+                           ? mozilla::dom::NOT_FROM_PARSER
+                           : aFromParser),
+        // Fragment parser-created scripts (if executable)
+        // behave like script-created scripts.
+        mCreatorParser(nullptr) {}
+
+  /**
+   * Content type identifying the scripting language. Can be empty, in
+   * which case javascript will be assumed.
+   * Return false if type attribute is not found.
+   */
+  virtual bool GetScriptType(nsAString& type) = 0;
+
+  /**
+   * Location of script source text. Can return null, in which case
+   * this is assumed to be an inline script element.
+   */
+  nsIURI* GetScriptURI() {
+    MOZ_ASSERT(mFrozen, "Not ready for this call yet!");
+    return mUri;
+  }
+
+  nsIPrincipal* GetScriptURITriggeringPrincipal() {
+    MOZ_ASSERT(mFrozen, "Not ready for this call yet!");
+    return mSrcTriggeringPrincipal;
+  }
+
+  /**
+   * Script source text for inline script elements.
+   */
+  virtual void GetScriptText(nsAString& text) const = 0;
+
+  virtual void GetScriptCharset(nsAString& charset) = 0;
+
+  /**
+   * Freezes the return values of the following methods so that subsequent
+   * modifications to the attributes don't change execution behavior:
+   *  - GetScriptIsModule()
+   *  - GetScriptIsImportMap()
+   *  - GetScriptDeferred()
+   *  - GetScriptAsync()
+   *  - GetScriptURI()
+   *  - GetScriptExternal()
+   */
+  virtual void FreezeExecutionAttrs(const mozilla::dom::Document*) = 0;
+
+  /**
+   * Is the script a module script.
+   */
+  bool GetScriptIsModule() {
+    MOZ_ASSERT(mFrozen, "Not ready for this call yet!");
+    return mKind == JS::loader::ScriptKind::eModule;
+  }
+
+  /**
+   * Is the script an import map.
+   */
+  bool GetScriptIsImportMap() {
+    MOZ_ASSERT(mFrozen, "Not ready for this call yet!");
+    return mKind == JS::loader::ScriptKind::eImportMap;
+  }
+
+  /**
+   * Is the script deferred.
+   */
+  bool GetScriptDeferred() {
+    MOZ_ASSERT(mFrozen, "Not ready for this call yet!");
+    return mDefer;
+  }
+
+  /**
+   * Is the script async.
+   */
+  bool GetScriptAsync() {
+    MOZ_ASSERT(mFrozen, "Not ready for this call yet!");
+    return mAsync;
+  }
+
+  /**
+   * Is the script an external script?
+   */
+  bool GetScriptExternal() {
+    MOZ_ASSERT(mFrozen, "Not ready for this call yet!");
+    return mExternal;
+  }
+
+  /**
+   * Returns how the element was created.
+   */
+  mozilla::dom::FromParser GetParserCreated() { return mParserCreated; }
+
+  void SetScriptLineNumber(uint32_t aLineNumber) { mLineNumber = aLineNumber; }
+
+  uint32_t GetScriptLineNumber() { return mLineNumber; }
+
+  void SetScriptColumnNumber(JS::ColumnNumberOneOrigin aColumnNumber) {
+    mColumnNumber = aColumnNumber;
+  }
+
+  JS::ColumnNumberOneOrigin GetScriptColumnNumber() { return mColumnNumber; }
+
+  void SetIsMalformed() { mMalformed = true; }
+
+  bool IsMalformed() { return mMalformed; }
+
+  void PreventExecution() { mAlreadyStarted = true; }
+
+  void LoseParserInsertedness() {
+    mUri = nullptr;
+    mCreatorParser = nullptr;
+    mParserCreated = mozilla::dom::NOT_FROM_PARSER;
+    mForceAsync = !GetAsyncState();
+
+    // Reset state set by FreezeExecutionAttrs().
+    mFrozen = false;
+    mExternal = false;
+    mAsync = false;
+    mDefer = false;
+    mKind = JS::loader::ScriptKind::eClassic;
+  }
+
+  void SetCreatorParser(nsIParser* aParser);
+
+  /**
+   * Unblocks the creator parser
+   */
+  void UnblockParser();
+
+  /**
+   * Attempts to resume parsing asynchronously
+   */
+  void ContinueParserAsync();
+
+  /**
+   * Informs the creator parser that the evaluation of this script is starting
+   */
+  void BeginEvaluating();
+
+  /**
+   * Informs the creator parser that the evaluation of this script is ending
+   */
+  void EndEvaluating();
+
+  /**
+   * Retrieves a pointer to the creator parser if this has one or null if not
+   */
+  already_AddRefed<nsIParser> GetCreatorParser();
+
+  /**
+   * This method is called when the parser finishes creating the script
+   * element's children, if any are present.
+   *
+   * @return whether the parser will be blocked while this script is being
+   *         loaded
+   */
+  bool AttemptToExecute() {
+    mDoneAddingChildren = true;
+    bool block = MaybeProcessScript();
+    if (!mAlreadyStarted) {
+      // Need to lose parser-insertedness here to allow another script to cause
+      // execution later.
+      LoseParserInsertedness();
+    }
+    return block;
+  }
+
+  /**
+   * Get the CORS mode of the script element
+   */
+  virtual mozilla::CORSMode GetCORSMode() const {
+    /* Default to no CORS */
+    return mozilla::CORS_NONE;
+  }
+
+  /**
+   * Get the fetch priority
+   * (https://html.spec.whatwg.org/multipage/scripting.html#attr-script-fetchpriority)
+   * of the script element.
+   */
+  virtual mozilla::dom::FetchPriority GetFetchPriority() const = 0;
+
+  /**
+   * Get referrer policy of the script element
+   */
+  virtual mozilla::dom::ReferrerPolicy GetReferrerPolicy();
+
+  /**
+   * Fire an error event
+   */
+  virtual nsresult FireErrorEvent() = 0;
+
+ protected:
+  /**
+   * Processes the script if it's in the document-tree and links to or
+   * contains a script. Once it has been evaluated there is no way to make it
+   * reevaluate the script, you'll have to create a new element. This also means
+   * that when adding a src attribute to an element that already contains an
+   * inline script, the script referenced by the src attribute will not be
+   * loaded.
+   *
+   * In order to be able to use multiple childNodes, or to use the
+   * fallback mechanism of using both inline script and linked script you have
+   * to add all attributes and childNodes before adding the element to the
+   * document-tree.
+   *
+   * @return whether the parser will be blocked while this script is being
+   *         loaded
+   */
+  virtual bool MaybeProcessScript() = 0;
+
+  /**
+   * Since we've removed the XPCOM interface to HTML elements, we need a way to
+   * retreive async state from script elements without bringing the type in.
+   */
+  virtual bool GetAsyncState() = 0;
+
+  /**
+   * Allow implementing elements to avoid unnecessary QueryReferences.
+   */
+  virtual nsIContent* GetAsContent() = 0;
+
+  /**
+   * Determine whether this is a(n) classic/module/importmap script.
+   */
+  void DetermineKindFromType(const mozilla::dom::Document* aOwnerDoc);
+
+  bool IsClassicNonAsyncDefer();
+
+  /**
+   * The start line number of the script.
+   */
+  uint32_t mLineNumber;
+
+  /**
+   * The start column number of the script.
+   */
+  JS::ColumnNumberOneOrigin mColumnNumber;
+
+  /**
+   * The "already started" flag per HTML5.
+   */
+  bool mAlreadyStarted;
+
+  /**
+   * The script didn't have an end tag.
+   */
+  bool mMalformed;
+
+  /**
+   * False if parser-inserted but the parser hasn't triggered running yet.
+   */
+  bool mDoneAddingChildren;
+
+  /**
+   * If true, the .async property returns true instead of reflecting the
+   * content attribute.
+   */
+  bool mForceAsync;
+
+  /**
+   * Whether src, defer and async are frozen.
+   */
+  bool mFrozen;
+
+  /**
+   * The effective deferredness.
+   */
+  bool mDefer;
+
+  /**
+   * The effective asyncness.
+   */
+  bool mAsync;
+
+  /**
+   * The effective externalness. A script can be external with mUri being null
+   * if the src attribute contained an invalid URL string.
+   */
+  bool mExternal;
+
+  /**
+   * The effective script kind.
+   */
+  JS::loader::ScriptKind mKind;
+
+  /**
+   * Whether this element was parser-created.
+   */
+  mozilla::dom::FromParser mParserCreated;
+
+  /**
+   * The effective src (or null if no src).
+   */
+  nsCOMPtr<nsIURI> mUri;
+
+  /**
+   * The triggering principal for the src URL.
+   */
+  nsCOMPtr<nsIPrincipal> mSrcTriggeringPrincipal;
+
+  /**
+   * The creator parser of a non-defer, non-async parser-inserted script.
+   */
+  nsWeakPtr mCreatorParser;
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(nsIScriptElement, NS_ISCRIPTELEMENT_IID)
+
+#endif  // nsIScriptElement_h___
diff --git a/dom/script/nsIScriptLoaderObserver.idl b/dom/script/nsIScriptLoaderObserver.idl
new file mode 100644
index 0000000000..ceeb79cb39
--- /dev/null
+++ b/dom/script/nsIScriptLoaderObserver.idl
@@ -0,0 +1,48 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsIScriptElement;
+interface nsIURI;
+
+[scriptable, uuid(7b787204-76fb-4764-96f1-fb7a666db4f4)]
+interface nsIScriptLoaderObserver : nsISupports {
+
+  /**
+   * The script is available for evaluation. For inline scripts, this
+   * method will be called synchronously. For externally loaded scripts,
+   * this method will be called when the load completes.
+   *
+   * @param aResult A result code representing the result of loading
+   *        a script. If this is a failure code, script evaluation
+   *        will not occur.
+   * @param aElement The element being processed.
+   * @param aIsInline Is this an inline classic script (as opposed to an
+   *        externally loaded classic script or module script)?
+   * @param aURI What is the URI of the script (the document URI if
+   *        it is inline).
+   * @param aLineNo At what line does the script appear (generally 1
+   *        if it is a loaded script).
+   */
+  void scriptAvailable(in nsresult aResult,
+                       in nsIScriptElement aElement,
+                       in boolean aIsInlineClassicScript,
+                       in nsIURI aURI,
+                       in uint32_t aLineNo);
+
+  /**
+   * The script has been evaluated.
+   *
+   * @param aResult A result code representing the success or failure of
+   *        the script evaluation.
+   * @param aElement The element being processed.
+   * @param aIsInline Is this an inline script or externally loaded?
+   */
+  [can_run_script]
+  void scriptEvaluated(in nsresult aResult,
+                       in nsIScriptElement aElement,
+                       in boolean aIsInline);
+};