diff --git a/content/browser/back_forward_cache_browsertest.cc b/content/browser/back_forward_cache_browsertest.cc
index 5f5a017a5908f4..8f74ac741a33e6 100644
--- a/content/browser/back_forward_cache_browsertest.cc
+++ b/content/browser/back_forward_cache_browsertest.cc
@@ -2528,7 +2528,7 @@ IN_PROC_BROWSER_TEST_F(
   web_contents()->GetController().GoBack();
   EXPECT_TRUE(WaitForLoadStop(shell()->web_contents()));
 
-  if (AreAllSitesIsolatedForTesting()) {
+  if (AreStrictSiteInstancesEnabled()) {
     ExpectNotRestored(
         {BackForwardCacheMetrics::NotRestoredReason::
              kRelatedActiveContentsExist,
@@ -4240,8 +4240,7 @@ IN_PROC_BROWSER_TEST_F(BackForwardCacheBrowserTest, CacheHTTPDocumentOnly) {
     // On Android, navigations to about:blank keeps the same RenderFrameHost.
     // Obviously, it can't enter the BackForwardCache, because it is still used
     // to display the current document.
-    if (test_case.url == blank_url &&
-        !SiteIsolationPolicy::UseDedicatedProcessesForAllSites()) {
+    if (test_case.url == blank_url && !AreStrictSiteInstancesEnabled()) {
       EXPECT_FALSE(delete_observer.deleted());
       EXPECT_FALSE(rfh->IsInBackForwardCache());
       EXPECT_EQ(rfh, current_frame_host());
@@ -5060,13 +5059,13 @@ IN_PROC_BROWSER_TEST_F(
   RenderFrameHostImpl* rfh_a1 = current_frame_host();
   RenderFrameHostImpl* rfh_b = rfh_a1->child_at(0)->current_frame_host();
   EXPECT_TRUE(ExecJs(rfh_b, "window.onunload = () => {} "));
-  EXPECT_EQ(AreAllSitesIsolatedForTesting(),
+  EXPECT_EQ(AreStrictSiteInstancesEnabled(),
             rfh_a1->GetSiteInstance() != rfh_b->GetSiteInstance());
 
   // 2) Navigate to A2.
   EXPECT_TRUE(NavigateToURL(shell(), url_a2));
   RenderFrameHostImpl* rfh_a2 = current_frame_host();
-  if (AreAllSitesIsolatedForTesting()) {
+  if (AreStrictSiteInstancesEnabled()) {
     // We should swap RFH & BIs and A1 should be in the back-forward cache.
     EXPECT_NE(rfh_a1, rfh_a2);
     EXPECT_FALSE(rfh_a1->GetSiteInstance()->IsRelatedSiteInstance(
diff --git a/content/browser/isolated_origin_browsertest.cc b/content/browser/isolated_origin_browsertest.cc
index da27a0335a2c88..88731fd4db8d14 100644
--- a/content/browser/isolated_origin_browsertest.cc
+++ b/content/browser/isolated_origin_browsertest.cc
@@ -2019,14 +2019,24 @@ IN_PROC_BROWSER_TEST_F(IsolatedOriginTest,
             child->current_frame_host()->GetSiteInstance()->GetSiteURL());
 
   // Navigate the child frame cross-site, but to a non-isolated origin. When
-  // not in --site-per-process, this should bring the subframe back into the
-  // main frame's SiteInstance.
+  // strict SiteInstaces are not enabled, this should bring the subframe back
+  // into the main frame's SiteInstance. If strict SiteInstances are enabled,
+  // we expect the SiteInstances to be different because a SiteInstance is not
+  // allowed to contain multiple sites in that mode. In all cases though we
+  // expect the navigation to end up in the same process.
   GURL bar_url(embedded_test_server()->GetURL("bar.com", "/title1.html"));
   EXPECT_FALSE(IsIsolatedOrigin(bar_url));
   NavigateIframeToURL(web_contents(), "test_iframe", bar_url);
-  EXPECT_EQ(web_contents()->GetSiteInstance(),
-            child->current_frame_host()->GetSiteInstance());
-  EXPECT_FALSE(child->current_frame_host()->IsCrossProcessSubframe());
+
+  if (AreStrictSiteInstancesEnabled()) {
+    EXPECT_NE(web_contents()->GetSiteInstance(),
+              child->current_frame_host()->GetSiteInstance());
+  } else {
+    EXPECT_EQ(web_contents()->GetSiteInstance(),
+              child->current_frame_host()->GetSiteInstance());
+  }
+  EXPECT_EQ(web_contents()->GetSiteInstance()->GetProcess(),
+            child->current_frame_host()->GetSiteInstance()->GetProcess());
 }
 
 // Check that a new isolated origin subframe will attempt to reuse an existing
@@ -2125,12 +2135,13 @@ IN_PROC_BROWSER_TEST_F(IsolatedOriginTest,
   FrameTreeNode* child = root->child_at(0);
 
   // Navigate iframe cross-site, but not to an isolated origin.  This should
-  // stay in the main frame's SiteInstance, unless we're in --site-per-process
-  // mode.  (Note that the bug for which this test is written is exclusive to
-  // --isolate-origins and does not happen with --site-per-process.)
+  // stay in the main frame's SiteInstance, unless we're in a strict
+  // SiteInstance mode (including --site-per-process). (Note that the bug for
+  // which this test is written is exclusive to --isolate-origins and does not
+  // happen with --site-per-process.)
   GURL bar_url(embedded_test_server()->GetURL("bar.com", "/title1.html"));
   NavigateIframeToURL(web_contents(), "test_iframe", bar_url);
-  if (AreAllSitesIsolatedForTesting()) {
+  if (AreStrictSiteInstancesEnabled()) {
     EXPECT_NE(root->current_frame_host()->GetSiteInstance(),
               child->current_frame_host()->GetSiteInstance());
   } else {
@@ -2236,8 +2247,7 @@ IN_PROC_BROWSER_TEST_F(IsolatedOriginTest,
     // At this point, the popup and the opener should still be in separate
     // SiteInstances.
     EXPECT_NE(newshell_site_instance_impl, root_site_instance_impl);
-    EXPECT_NE(AreAllSitesIsolatedForTesting(),
-              newshell_site_instance_impl->IsDefaultSiteInstance());
+    EXPECT_FALSE(newshell_site_instance_impl->IsDefaultSiteInstance());
     EXPECT_FALSE(root_site_instance_impl->IsDefaultSiteInstance());
   }
 
@@ -3060,7 +3070,11 @@ IN_PROC_BROWSER_TEST_F(IsolatedOriginLongListTest, Test) {
   if (!AreAllSitesIsolatedForTesting()) {
     EXPECT_EQ(main_frame->GetProcess()->GetID(),
               subframe3->GetProcess()->GetID());
-    EXPECT_EQ(main_frame->GetSiteInstance(), subframe3->GetSiteInstance());
+    if (AreStrictSiteInstancesEnabled()) {
+      EXPECT_NE(main_frame->GetSiteInstance(), subframe3->GetSiteInstance());
+    } else {
+      EXPECT_EQ(main_frame->GetSiteInstance(), subframe3->GetSiteInstance());
+    }
   }
 
   // isolated.foo.com and foo999.com are on the list of origins to isolate -
@@ -3115,7 +3129,7 @@ IN_PROC_BROWSER_TEST_F(IsolatedOriginTest, SubframeErrorPages) {
     observer.Wait();
     EXPECT_EQ(child2->current_url(), regular_url);
     EXPECT_TRUE(handle_observer.is_error());
-    if (AreAllSitesIsolatedForTesting()) {
+    if (AreStrictSiteInstancesEnabled()) {
       EXPECT_NE(root->current_frame_host()->GetSiteInstance(),
                 child2->current_frame_host()->GetSiteInstance());
       EXPECT_EQ(SiteInstance::GetSiteForURL(web_contents()->GetBrowserContext(),
@@ -3215,15 +3229,11 @@ IN_PROC_BROWSER_TEST_F(IsolatedOriginTest, AIsolatedCA) {
 
     EXPECT_TRUE(HasDefaultSiteInstance(a));
     EXPECT_FALSE(HasDefaultSiteInstance(b));
-  } else {
-    // Documenting current behavior where the top level document doesn't end
-    // up in a default SiteInstance even though it is not isolated and does not
-    // require a dedicated process. c.com does get placed in a default
-    // SiteInstance because we currently allow subframes that don't require
-    // isolation to share a process. This behavior should go away once we
-    // turn on default SiteInstances by default.
+  } else if (AreStrictSiteInstancesEnabled()) {
+    // All sites have their own SiteInstance and sites that are not isolated
+    // are all placed in the same process.
     EXPECT_NE(a->GetProcess()->GetID(), b->GetProcess()->GetID());
-    EXPECT_NE(a->GetProcess()->GetID(), c->GetProcess()->GetID());
+    EXPECT_EQ(a->GetProcess()->GetID(), c->GetProcess()->GetID());
 
     EXPECT_NE(a->GetSiteInstance(), b->GetSiteInstance());
     EXPECT_NE(a->GetSiteInstance(), c->GetSiteInstance());
@@ -3232,7 +3242,9 @@ IN_PROC_BROWSER_TEST_F(IsolatedOriginTest, AIsolatedCA) {
 
     EXPECT_FALSE(HasDefaultSiteInstance(a));
     EXPECT_FALSE(HasDefaultSiteInstance(b));
-    EXPECT_TRUE(HasDefaultSiteInstance(c));
+    EXPECT_FALSE(HasDefaultSiteInstance(c));
+  } else {
+    FAIL() << "Unexpected process model configuration.";
   }
 }
 
@@ -3466,12 +3478,15 @@ IN_PROC_BROWSER_TEST_F(DynamicIsolatedOriginTest,
 
   // The two frames should be in the same process, since neither site is
   // isolated so far.
-  if (!AreAllSitesIsolatedForTesting()) {
+  if (AreStrictSiteInstancesEnabled()) {
+    EXPECT_NE(root->current_frame_host()->GetSiteInstance(),
+              child->current_frame_host()->GetSiteInstance());
+  } else {
     EXPECT_EQ(root->current_frame_host()->GetSiteInstance(),
               child->current_frame_host()->GetSiteInstance());
-    EXPECT_EQ(root->current_frame_host()->GetProcess(),
-              child->current_frame_host()->GetProcess());
   }
+  EXPECT_EQ(root->current_frame_host()->GetProcess(),
+            child->current_frame_host()->GetProcess());
 
   // Start isolating foo.com.
   auto* policy = ChildProcessSecurityPolicyImpl::GetInstance();
@@ -3493,12 +3508,23 @@ IN_PROC_BROWSER_TEST_F(DynamicIsolatedOriginTest,
       "bar.com", "/cross_site_iframe_factory.html?bar.com(foo.com)"));
   NavigateIframeToURL(web_contents(), "test_iframe", bar_with_foo_url);
   FrameTreeNode* grandchild = child->child_at(0);
-  EXPECT_EQ(root->current_frame_host()->GetSiteInstance(),
-            child->current_frame_host()->GetSiteInstance());
-  EXPECT_EQ(child->current_frame_host()->GetSiteInstance(),
-            grandchild->current_frame_host()->GetSiteInstance());
+  if (AreStrictSiteInstancesEnabled()) {
+    EXPECT_NE(root->current_frame_host()->GetSiteInstance(),
+              child->current_frame_host()->GetSiteInstance());
+    EXPECT_NE(child->current_frame_host()->GetSiteInstance(),
+              grandchild->current_frame_host()->GetSiteInstance());
+  } else {
+    EXPECT_EQ(root->current_frame_host()->GetSiteInstance(),
+              child->current_frame_host()->GetSiteInstance());
+    EXPECT_EQ(child->current_frame_host()->GetSiteInstance(),
+              grandchild->current_frame_host()->GetSiteInstance());
+  }
   EXPECT_EQ(root->current_frame_host()->GetSiteInstance(),
             grandchild->current_frame_host()->GetSiteInstance());
+  EXPECT_EQ(root->current_frame_host()->GetProcess(),
+            child->current_frame_host()->GetProcess());
+  EXPECT_EQ(child->current_frame_host()->GetProcess(),
+            grandchild->current_frame_host()->GetProcess());
 
   // Create an unrelated window, which will be in a new BrowsingInstance.
   // Ensure that foo.com becomes an isolated origin in that window.  A
@@ -3773,8 +3799,14 @@ IN_PROC_BROWSER_TEST_F(DynamicIsolatedOriginTest,
   EXPECT_EQ(child->current_url(), bar_url);
 
   // The iframe should not be in an OOPIF yet.
-  EXPECT_EQ(root->current_frame_host()->GetSiteInstance(),
-            child->current_frame_host()->GetSiteInstance());
+  if (AreStrictSiteInstancesEnabled()) {
+    EXPECT_NE(root->current_frame_host()->GetSiteInstance(),
+              child->current_frame_host()->GetSiteInstance());
+
+  } else {
+    EXPECT_EQ(root->current_frame_host()->GetSiteInstance(),
+              child->current_frame_host()->GetSiteInstance());
+  }
   EXPECT_EQ(root->current_frame_host()->GetProcess(),
             child->current_frame_host()->GetProcess());
 
@@ -3959,8 +3991,13 @@ IN_PROC_BROWSER_TEST_F(DynamicIsolatedOriginTest, PerProfileIsolation) {
   root = web_contents()->GetFrameTree()->root();
   child = root->child_at(0);
   EXPECT_EQ(child->current_url(), bar_url);
-  EXPECT_EQ(root->current_frame_host()->GetSiteInstance(),
-            child->current_frame_host()->GetSiteInstance());
+  if (AreStrictSiteInstancesEnabled()) {
+    EXPECT_NE(root->current_frame_host()->GetSiteInstance(),
+              child->current_frame_host()->GetSiteInstance());
+  } else {
+    EXPECT_EQ(root->current_frame_host()->GetSiteInstance(),
+              child->current_frame_host()->GetSiteInstance());
+  }
   EXPECT_EQ(root->current_frame_host()->GetProcess(),
             child->current_frame_host()->GetProcess());
 }
@@ -3982,7 +4019,12 @@ IN_PROC_BROWSER_TEST_F(DynamicIsolatedOriginTest, ForceBrowsingInstanceSwap) {
   FrameTreeNode* child = root->child_at(0);
   scoped_refptr<SiteInstance> first_instance =
       root->current_frame_host()->GetSiteInstance();
-  EXPECT_EQ(first_instance, child->current_frame_host()->GetSiteInstance());
+
+  if (AreStrictSiteInstancesEnabled()) {
+    EXPECT_NE(first_instance, child->current_frame_host()->GetSiteInstance());
+  } else {
+    EXPECT_EQ(first_instance, child->current_frame_host()->GetSiteInstance());
+  }
   EXPECT_EQ(root->current_frame_host()->GetProcess(),
             child->current_frame_host()->GetProcess());
   auto* policy = ChildProcessSecurityPolicyImpl::GetInstance();
diff --git a/content/browser/navigation_browsertest.cc b/content/browser/navigation_browsertest.cc
index 5c4ff95a345ef3..f6add29f1614a0 100644
--- a/content/browser/navigation_browsertest.cc
+++ b/content/browser/navigation_browsertest.cc
@@ -501,13 +501,13 @@ IN_PROC_BROWSER_TEST_F(NavigationBrowserTest,
     EXPECT_EQ(initiator_process_id, observer.last_initiator_process_id());
   }
 
-  // The RenderFrameHost should not have changed unless site-per-process or
-  // proactive BrowsingInstance swap is enabled.
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
-    EXPECT_NE(initial_rfh, current_frame_host());
-  } else {
+  // The RenderFrameHost should have changed unless default SiteInstances
+  // are enabled and proactive BrowsingInstance swaps are disabled.
+  if (AreDefaultSiteInstancesEnabled() &&
+      !CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
     EXPECT_EQ(initial_rfh, current_frame_host());
+  } else {
+    EXPECT_NE(initial_rfh, current_frame_host());
   }
 }
 
diff --git a/content/browser/renderer_host/navigation_controller_impl_browsertest.cc b/content/browser/renderer_host/navigation_controller_impl_browsertest.cc
index e168413aeb2c10..fd5f07f6ee9b1e 100644
--- a/content/browser/renderer_host/navigation_controller_impl_browsertest.cc
+++ b/content/browser/renderer_host/navigation_controller_impl_browsertest.cc
@@ -4593,7 +4593,7 @@ IN_PROC_BROWSER_TEST_P(NavigationControllerBrowserTest,
   FrameTreeNode* root = static_cast<WebContentsImpl*>(shell()->web_contents())
                             ->GetFrameTree()
                             ->root();
-  SiteInstance* main_site_instance =
+  SiteInstanceImpl* main_site_instance =
       root->current_frame_host()->GetSiteInstance();
 
   // The main frame defaults to an empty name.
@@ -4656,14 +4656,15 @@ IN_PROC_BROWSER_TEST_P(NavigationControllerBrowserTest,
   EXPECT_TRUE(NavigateToURLFromRenderer(foo_subframe, bar_url));
   EXPECT_TRUE(WaitForLoadStop(shell()->web_contents()));
 
-  // When run just with subframe navigation entries enabled and not in
-  // site-per-process-mode the subframe should be in the same SiteInstance as
-  // its parent.
-  if (!AreAllSitesIsolatedForTesting()) {
-    EXPECT_EQ(main_site_instance,
+  if (AreStrictSiteInstancesEnabled()) {
+    EXPECT_NE(main_site_instance,
               foo_subframe->current_frame_host()->GetSiteInstance());
   } else {
-    EXPECT_NE(main_site_instance,
+    // When strict SiteInstances are not being used, the subframe should be
+    // the same as its parent because both sites get routed to the default
+    // SiteInstance.
+    EXPECT_TRUE(main_site_instance->IsDefaultSiteInstance());
+    EXPECT_EQ(main_site_instance,
               foo_subframe->current_frame_host()->GetSiteInstance());
   }
 
@@ -5518,12 +5519,16 @@ IN_PROC_BROWSER_TEST_P(NavigationControllerBrowserTest,
   EXPECT_EQ(main_url_a, new_root->current_url());
   EXPECT_EQ(frame_url_b, new_root->child_at(0)->current_url());
 
-  // The subframe should only be in a different SiteInstance if OOPIFs are
-  // required for all sites.
-  if (AreAllSitesIsolatedForTesting()) {
+  if (AreStrictSiteInstancesEnabled()) {
     EXPECT_NE(new_root->current_frame_host()->GetSiteInstance(),
               new_root->child_at(0)->current_frame_host()->GetSiteInstance());
   } else {
+    // When strict SiteInstances are not enabled, the subframe should be in the
+    // same SiteInstance as the parent because both sites get mapped to the
+    // default SiteInstance.
+    EXPECT_TRUE(new_root->current_frame_host()
+                    ->GetSiteInstance()
+                    ->IsDefaultSiteInstance());
     EXPECT_EQ(new_root->current_frame_host()->GetSiteInstance(),
               new_root->child_at(0)->current_frame_host()->GetSiteInstance());
   }
@@ -5960,9 +5965,9 @@ class RenderProcessKilledObserver : public WebContentsObserver {
 // not modify the underlying last committed entry.)  Not crashing means that
 // the test is successful.
 IN_PROC_BROWSER_TEST_P(NavigationControllerBrowserTest, ReloadOriginalRequest) {
-  // TODO(lukasza): https://crbug.com/417518: Get tests working with
-  // --site-per-process.
-  if (SiteIsolationPolicy::UseDedicatedProcessesForAllSites() ||
+  // TODO(lukasza): https://crbug.com/1159466: Get tests working for all
+  // process model modes.
+  if (AreStrictSiteInstancesEnabled() ||
       CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
     return;
   }
diff --git a/content/browser/renderer_host/navigator_unittest.cc b/content/browser/renderer_host/navigator_unittest.cc
index a1432ab3bd0ed1..0ba339241fec82 100644
--- a/content/browser/renderer_host/navigator_unittest.cc
+++ b/content/browser/renderer_host/navigator_unittest.cc
@@ -40,6 +40,19 @@
 
 namespace content {
 
+namespace {
+
+// Helper function that determines if a test should expect a cross-site
+// navigation to trigger a SiteInstance change based on the current process
+// model.
+bool ExpectSiteInstanceChange(SiteInstanceImpl* site_instance) {
+  return AreAllSitesIsolatedForTesting() ||
+         CanCrossSiteNavigationsProactivelySwapBrowsingInstances() ||
+         !site_instance->IsDefaultSiteInstance();
+}
+
+}  // namespace
+
 class NavigatorTest : public RenderViewHostImplTestHarness {
  public:
   using SiteInstanceDescriptor = RenderFrameHostManager::SiteInstanceDescriptor;
@@ -190,7 +203,10 @@ TEST_F(NavigatorTest, SimpleRendererInitiatedCrossSiteNavigation) {
 
   contents()->NavigateAndCommit(kUrl1);
   EXPECT_TRUE(main_test_rfh()->IsRenderFrameLive());
-  int32_t site_instance_id_1 = main_test_rfh()->GetSiteInstance()->GetId();
+  scoped_refptr<SiteInstanceImpl> site_instance_1 =
+      main_test_rfh()->GetSiteInstance();
+  bool expect_site_instance_change =
+      ExpectSiteInstanceChange(site_instance_1.get());
 
   // Start a renderer-initiated navigation.
   EXPECT_FALSE(main_test_rfh()->navigation_request());
@@ -206,8 +222,7 @@ TEST_F(NavigatorTest, SimpleRendererInitiatedCrossSiteNavigation) {
   EXPECT_EQ(NavigationRequest::WILL_START_REQUEST, request->state());
   EXPECT_EQ(kUrl2, request->common_params().url);
   EXPECT_FALSE(request->browser_initiated());
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
+  if (expect_site_instance_change) {
     EXPECT_TRUE(GetSpeculativeRenderFrameHost(node));
   } else {
     EXPECT_FALSE(GetSpeculativeRenderFrameHost(node));
@@ -215,8 +230,7 @@ TEST_F(NavigatorTest, SimpleRendererInitiatedCrossSiteNavigation) {
 
   // Have the current RenderFrameHost commit the navigation.
   navigation->ReadyToCommit();
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
+  if (expect_site_instance_change) {
     EXPECT_EQ(navigation->GetFinalRenderFrameHost(),
               GetSpeculativeRenderFrameHost(node));
   }
@@ -228,12 +242,14 @@ TEST_F(NavigatorTest, SimpleRendererInitiatedCrossSiteNavigation) {
   EXPECT_EQ(kUrl2, contents()->GetLastCommittedURL());
   EXPECT_FALSE(GetSpeculativeRenderFrameHost(node));
 
-  // The SiteInstance did not change unless site-per-process is enabled.
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
-    EXPECT_NE(site_instance_id_1, main_test_rfh()->GetSiteInstance()->GetId());
+  if (expect_site_instance_change) {
+    EXPECT_NE(site_instance_1->GetId(),
+              main_test_rfh()->GetSiteInstance()->GetId());
+    EXPECT_EQ(site_instance_1->IsDefaultSiteInstance(),
+              main_test_rfh()->GetSiteInstance()->IsDefaultSiteInstance());
   } else {
-    EXPECT_EQ(site_instance_id_1, main_test_rfh()->GetSiteInstance()->GetId());
+    EXPECT_EQ(site_instance_1->GetId(),
+              main_test_rfh()->GetSiteInstance()->GetId());
   }
 }
 
@@ -315,9 +331,11 @@ TEST_F(NavigatorTest, BeginNavigation) {
   EXPECT_FALSE(GetSpeculativeRenderFrameHost(root_node));
 
   // Subframe navigations should never create a speculative RenderFrameHost,
-  // unless site-per-process is enabled. In that case, as the subframe
-  // navigation is to a different site and is still ongoing, it should have one.
-  if (AreAllSitesIsolatedForTesting()) {
+  // unless site-per-process or ProcessSharingWithStrictSiteInstances is
+  // enabled. In that case, as the subframe navigation is to a different site
+  // and is still ongoing, it should have one.
+  bool expect_site_instance_change = AreStrictSiteInstancesEnabled();
+  if (expect_site_instance_change) {
     EXPECT_TRUE(GetSpeculativeRenderFrameHost(subframe_node));
   } else {
     EXPECT_FALSE(GetSpeculativeRenderFrameHost(subframe_node));
@@ -359,7 +377,7 @@ TEST_F(NavigatorTest, BeginNavigation) {
 
   // As the main frame hasn't yet committed the subframe still exists. Thus, the
   // above situation regarding subframe navigations is valid here.
-  if (AreAllSitesIsolatedForTesting()) {
+  if (expect_site_instance_change) {
     EXPECT_TRUE(GetSpeculativeRenderFrameHost(subframe_node));
   } else {
     EXPECT_FALSE(GetSpeculativeRenderFrameHost(subframe_node));
@@ -598,6 +616,8 @@ TEST_F(NavigatorTest, RendererUserInitiatedNavigationCancel) {
   // Initialization.
   contents()->NavigateAndCommit(kUrl0);
   FrameTreeNode* node = main_test_rfh()->frame_tree_node();
+  bool expect_site_instance_change =
+      ExpectSiteInstanceChange(main_test_rfh()->GetSiteInstance());
 
   // Start a browser-initiated navigation to the 1st URL and invoke its
   // beforeUnload completion callback.
@@ -634,8 +654,7 @@ TEST_F(NavigatorTest, RendererUserInitiatedNavigationCancel) {
 
   // Confirm that the speculative RenderFrameHost was destroyed in the non
   // SitePerProcess case.
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
+  if (expect_site_instance_change) {
     EXPECT_TRUE(GetSpeculativeRenderFrameHost(node));
   } else {
     EXPECT_FALSE(GetSpeculativeRenderFrameHost(node));
@@ -661,6 +680,8 @@ TEST_F(NavigatorTest,
   // Initialization.
   contents()->NavigateAndCommit(kUrl0);
   FrameTreeNode* node = main_test_rfh()->frame_tree_node();
+  bool expect_site_instance_change =
+      ExpectSiteInstanceChange(main_test_rfh()->GetSiteInstance());
 
   // Start a renderer-initiated user-initiated navigation to the 1st URL.
   EXPECT_FALSE(main_test_rfh()->navigation_request());
@@ -674,8 +695,7 @@ TEST_F(NavigatorTest,
   EXPECT_EQ(kUrl1, request1->common_params().url);
   EXPECT_FALSE(request1->browser_initiated());
   EXPECT_TRUE(request1->common_params().has_user_gesture);
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
+  if (expect_site_instance_change) {
     EXPECT_TRUE(GetSpeculativeRenderFrameHost(node));
   } else {
     EXPECT_FALSE(GetSpeculativeRenderFrameHost(node));
@@ -695,8 +715,7 @@ TEST_F(NavigatorTest,
   EXPECT_EQ(kUrl2, request2->common_params().url);
   EXPECT_FALSE(request2->browser_initiated());
   EXPECT_FALSE(request2->common_params().has_user_gesture);
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
+  if (expect_site_instance_change) {
     EXPECT_TRUE(GetSpeculativeRenderFrameHost(node));
   } else {
     EXPECT_FALSE(GetSpeculativeRenderFrameHost(node));
@@ -762,6 +781,8 @@ TEST_F(NavigatorTest,
   contents()->NavigateAndCommit(kUrl0);
   FrameTreeNode* node = main_test_rfh()->frame_tree_node();
   int32_t site_instance_id_0 = main_test_rfh()->GetSiteInstance()->GetId();
+  bool expect_site_instance_change =
+      ExpectSiteInstanceChange(main_test_rfh()->GetSiteInstance());
 
   // Start a renderer-initiated non-user-initiated navigation to the 1st URL.
   EXPECT_FALSE(main_test_rfh()->navigation_request());
@@ -776,8 +797,7 @@ TEST_F(NavigatorTest,
   EXPECT_EQ(kUrl1, request1->common_params().url);
   EXPECT_FALSE(request1->browser_initiated());
   EXPECT_FALSE(request1->common_params().has_user_gesture);
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
+  if (expect_site_instance_change) {
     EXPECT_TRUE(GetSpeculativeRenderFrameHost(node));
   } else {
     EXPECT_FALSE(GetSpeculativeRenderFrameHost(node));
@@ -797,8 +817,7 @@ TEST_F(NavigatorTest,
   EXPECT_EQ(kUrl2, request2->common_params().url);
   EXPECT_FALSE(request2->browser_initiated());
   EXPECT_FALSE(request2->common_params().has_user_gesture);
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
+  if (expect_site_instance_change) {
     EXPECT_TRUE(GetSpeculativeRenderFrameHost(node));
   } else {
     EXPECT_FALSE(GetSpeculativeRenderFrameHost(node));
@@ -811,9 +830,7 @@ TEST_F(NavigatorTest,
   navigation2->Commit();
   EXPECT_EQ(kUrl2, contents()->GetLastCommittedURL());
 
-  // The SiteInstance did not change unless site-per-process is enabled.
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
+  if (expect_site_instance_change) {
     EXPECT_NE(site_instance_id_0, main_test_rfh()->GetSiteInstance()->GetId());
   } else {
     EXPECT_EQ(site_instance_id_0, main_test_rfh()->GetSiteInstance()->GetId());
@@ -1042,7 +1059,7 @@ TEST_F(NavigatorTest, SiteInstanceDescriptionConversion) {
       ChildProcessSecurityPolicy::IsolatedOriginSource::TEST,
       browser_context());
   contents()->NavigateAndCommit(kUrl1);
-  SiteInstance* current_instance = main_test_rfh()->GetSiteInstance();
+  SiteInstanceImpl* current_instance = main_test_rfh()->GetSiteInstance();
   ASSERT_TRUE(current_instance);
 
   // 1) Convert a descriptor pointing to the current instance.
@@ -1101,12 +1118,15 @@ TEST_F(NavigatorTest, SiteInstanceDescriptionConversion) {
     EXPECT_NE(current_instance, related_instance.get());
     EXPECT_NE(unrelated_instance.get(), related_instance.get());
 
-    if (AreAllSitesIsolatedForTesting()) {
-      EXPECT_EQ(SiteInstance::GetSiteForURL(browser_context(), kUrlSameSiteAs2),
-                related_instance->GetSiteURL());
+    auto* related_instance_impl =
+        static_cast<SiteInstanceImpl*>(related_instance.get());
+
+    if (AreDefaultSiteInstancesEnabled()) {
+      ASSERT_TRUE(related_instance_impl->IsDefaultSiteInstance());
     } else {
-      EXPECT_TRUE(static_cast<SiteInstanceImpl*>(related_instance.get())
-                      ->IsDefaultSiteInstance());
+      EXPECT_EQ(SiteInstanceImpl::ComputeSiteInfoForTesting(
+                    current_instance->GetIsolationContext(), kUrlSameSiteAs2),
+                related_instance_impl->GetSiteInfo());
     }
   }
 
diff --git a/content/browser/renderer_host/render_frame_host_impl_browsertest.cc b/content/browser/renderer_host/render_frame_host_impl_browsertest.cc
index e1673753110983..13bc06dd46d18f 100644
--- a/content/browser/renderer_host/render_frame_host_impl_browsertest.cc
+++ b/content/browser/renderer_host/render_frame_host_impl_browsertest.cc
@@ -791,14 +791,14 @@ IN_PROC_BROWSER_TEST_F(RenderFrameHostImplBeforeUnloadBrowserTest,
   EXPECT_EQ(main_frame, child->GetBeforeUnloadInitiator());
   EXPECT_EQ(main_frame, main_frame->GetBeforeUnloadInitiator());
 
-  // When in --site-per-process mode, LoadURL() should trigger two beforeunload
-  // IPCs for subframe and the main frame: the subframe has a beforeunload
-  // handler, and while the main frame does not, we always send the IPC to
-  // navigating frames, regardless of whether or not they have a handler.
+  // When in a strict SiteInstances mode, LoadURL() should trigger two
+  // beforeunload IPCs for subframe and the main frame: the subframe has a
+  // beforeunload handler, and while the main frame does not, we always send the
+  // IPC to navigating frames, regardless of whether or not they have a handler.
   //
-  // Without --site-per-process, only one beforeunload IPC should be sent to
+  // Without strict SiteInstances, only one beforeunload IPC should be sent to
   // the main frame, which will handle both (same-process) frames.
-  EXPECT_EQ(AreAllSitesIsolatedForTesting() ? 2u : 1u,
+  EXPECT_EQ(AreStrictSiteInstancesEnabled() ? 2u : 1u,
             main_frame->beforeunload_pending_replies_.size());
 
   // Wait for the beforeunload dialog to be shown from the subframe.
@@ -811,12 +811,12 @@ IN_PROC_BROWSER_TEST_F(RenderFrameHostImplBeforeUnloadBrowserTest,
   EXPECT_TRUE(main_frame->is_waiting_for_beforeunload_completion());
   EXPECT_FALSE(child->is_waiting_for_beforeunload_completion());
 
-  // In --site-per-process mode, the beforeunload completion callback should
-  // happen on the child RFH.  Without --site-per-process, it will come from the
-  // main frame RFH, which processes beforeunload for both main frame and child
-  // frame, since they are in the same process.
+  // In a strict SiteInstances mode, the beforeunload completion callback should
+  // happen on the child RFH.  Without strict SiteInstances, it will come from
+  // the main frame RFH, which processes beforeunload for both main frame and
+  // child frame, since they are in the same process and SiteInstance.
   RenderFrameHostImpl* frame_that_sent_beforeunload_ipc =
-      AreAllSitesIsolatedForTesting() ? child : main_frame;
+      AreStrictSiteInstancesEnabled() ? child : main_frame;
   EXPECT_TRUE(main_frame->beforeunload_pending_replies_.count(
       frame_that_sent_beforeunload_ipc));
 
diff --git a/content/browser/renderer_host/render_frame_host_manager_browsertest.cc b/content/browser/renderer_host/render_frame_host_manager_browsertest.cc
index da9f87698eb14b..6c151b9123522e 100644
--- a/content/browser/renderer_host/render_frame_host_manager_browsertest.cc
+++ b/content/browser/renderer_host/render_frame_host_manager_browsertest.cc
@@ -90,6 +90,13 @@ namespace content {
 
 namespace {
 
+// Helper function that return true in cases where the current process model
+// will return the same SiteInstance for a cross-process navigation.
+bool ExpectSameSiteInstance() {
+  return AreDefaultSiteInstancesEnabled() &&
+         !CanCrossSiteNavigationsProactivelySwapBrowsingInstances();
+}
+
 const char kOpenUrlViaClickTargetFunc[] =
     "(function(url) {\n"
     "  var lnk = document.createElement(\"a\");\n"
@@ -680,10 +687,10 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
   // Should have the same SiteInstance unless we're in site-per-process mode.
   scoped_refptr<SiteInstance> blank_site_instance(
       new_shell->web_contents()->GetSiteInstance());
-  if (AreAllSitesIsolatedForTesting())
-    EXPECT_NE(orig_site_instance, blank_site_instance);
-  else
+  if (AreDefaultSiteInstancesEnabled())
     EXPECT_EQ(orig_site_instance, blank_site_instance);
+  else
+    EXPECT_NE(orig_site_instance, blank_site_instance);
 }
 
 // Test for crbug.com/24447.  Following a cross-site link with rel=noreferrer
@@ -715,14 +722,12 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
   EXPECT_EQ("/title2.html",
             shell()->web_contents()->GetLastCommittedURL().path());
 
-  // Should have the same SiteInstance unless we're in site-per-process mode.
   scoped_refptr<SiteInstance> noref_site_instance(
       shell()->web_contents()->GetSiteInstance());
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
-    EXPECT_NE(orig_site_instance, noref_site_instance);
-  } else {
+  if (ExpectSameSiteInstance()) {
     EXPECT_EQ(orig_site_instance, noref_site_instance);
+  } else {
+    EXPECT_NE(orig_site_instance, noref_site_instance);
   }
 }
 
@@ -754,14 +759,12 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
   EXPECT_EQ("/title2.html",
             shell()->web_contents()->GetLastCommittedURL().path());
 
-  // Should have the same SiteInstance unless we're in site-per-process mode.
   scoped_refptr<SiteInstance> noref_site_instance(
       shell()->web_contents()->GetSiteInstance());
-  if (AreAllSitesIsolatedForTesting() ||
-      CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
-    EXPECT_NE(orig_site_instance, noref_site_instance);
-  } else {
+  if (ExpectSameSiteInstance()) {
     EXPECT_EQ(orig_site_instance, noref_site_instance);
+  } else {
+    EXPECT_NE(orig_site_instance, noref_site_instance);
   }
 }
 
@@ -840,7 +843,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
 IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest, MAYBE_DisownOpener) {
   StartEmbeddedServer();
 
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "foo.com" so we are guaranteed to get a non-default
     // SiteInstance for navigations to this origin.
     IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
@@ -940,7 +943,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest, DisownSubframeOpener) {
 IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
                        PreserveTopFrameWindowNameOnCrossProcessNavigations) {
   StartEmbeddedServer();
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "foo.com" so we are guaranteed it is placed in a different
     // process.
     IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
@@ -1008,7 +1011,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
 IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
                        SupportCrossProcessPostMessage) {
   StartEmbeddedServer();
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "foo.com" so we are guaranteed it is placed in a different
     // process.
     IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
@@ -1149,7 +1152,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
 IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
                        SupportCrossProcessPostMessageWithMessagePort) {
   StartEmbeddedServer();
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "foo.com" so we are guaranteed it is placed in a different
     // process.
     IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
@@ -1239,7 +1242,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
                        AllowTargetedNavigationsInOpenerAfterSwap) {
   StartEmbeddedServer();
 
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "foo.com" so we are guaranteed to get a non-default
     // SiteInstance for navigations to this origin.
     IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
@@ -1346,7 +1349,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
 IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
                        ProcessExitWithSwappedOutViews) {
   StartEmbeddedServer();
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "foo.com" so we are guaranteed to get a non-default
     // SiteInstance for navigations to this origin.
     IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
@@ -2349,7 +2352,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
                        DontPreemptNavigationWithFrameTreeUpdate) {
   StartEmbeddedServer();
 
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "foo.com" so we are guaranteed to get a non-default
     // SiteInstance for navigations to this origin.
     IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
@@ -3080,7 +3083,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
                        ReinitializeOpenerChainAfterCrashAndReload) {
   StartEmbeddedServer();
 
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "foo.com" so we are guaranteed to get a non-default
     // SiteInstance for navigations to this origin.
     IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
@@ -3149,7 +3152,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
 // the other popup, and ensure that the opener is updated in all processes.
 IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest, UpdateOpener) {
   StartEmbeddedServer();
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "foo.com" so we are guaranteed it is placed in a different
     // process.
     IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
@@ -3921,9 +3924,9 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest, CoReferencingFrames) {
 
   // The FrameTree contains two successful instances of each site plus an
   // unsuccessfully-navigated third instance of B with a blank URL.  When not in
-  // site-per-process mode, the FrameTreeVisualizer depicts all nodes as
+  // strict SiteInstance mode, the FrameTreeVisualizer depicts all nodes as
   // referencing Site A because iframes are identified with their root site.
-  if (AreAllSitesIsolatedForTesting()) {
+  if (AreStrictSiteInstancesEnabled()) {
     EXPECT_EQ(
         " Site A ------------ proxies for B\n"
         "   +--Site B ------- proxies for A\n"
@@ -4379,8 +4382,7 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
   // Since this is a browser-initiated, cross-site navigation, it will swap
   // BrowsingInstances, and create a new foo.com SiteInstance, distinct from
   // the initial one.
-  if (!AreAllSitesIsolatedForTesting() &&
-      !CanCrossSiteNavigationsProactivelySwapBrowsingInstances()) {
+  if (ExpectSameSiteInstance()) {
     EXPECT_EQ(site_instance, shell()->web_contents()->GetSiteInstance());
   } else {
     EXPECT_NE(site_instance, shell()->web_contents()->GetSiteInstance());
@@ -4785,10 +4787,10 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
                        ErrorPageNavigationReload_InSubframe_NetworkError) {
   StartEmbeddedServer();
 
-  // Isolating a.com helps more robustly exercise platforms without strict site
-  // isolation - we want to ensure that enforcing |initiator_origin| in
-  // BeginNavigation is compatible with process locks, even when only one of the
-  // frames requires isolation.
+  // Isolating a.com helps more robustly exercise platforms without strict
+  // site isolation - we want to ensure that enforcing |initiator_origin| in
+  // BeginNavigation is compatible with process locks, even when only one of
+  // the frames requires isolation.
   IsolateOriginsForTesting(embedded_test_server(), shell()->web_contents(),
                            {"b.com"});
 
@@ -4953,11 +4955,11 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
         child1->current_frame_host()->GetSiteInstance();
 
     GURL c_site_url = child1_site_instance->GetSiteURL();
-    if (AreAllSitesIsolatedForTesting()) {
+    if (AreDefaultSiteInstancesEnabled()) {
+      EXPECT_TRUE(child1_site_instance->IsDefaultSiteInstance());
+    } else {
       EXPECT_EQ("c.com", c_site_url.host());
       EXPECT_EQ(test_url.host(), c_site_url.host());
-    } else {
-      EXPECT_TRUE(child1_site_instance->IsDefaultSiteInstance());
     }
     EXPECT_NE(a_site_url, c_site_url);
     EXPECT_NE(b_site_url, c_site_url);
@@ -5749,13 +5751,15 @@ IN_PROC_BROWSER_TEST_P(
       static_cast<SiteInstanceImpl*>(
           web_contents->GetMainFrame()->GetSiteInstance());
 
-  // Check that A and B are in different BrowsingInstances (both are in default
-  // SiteInstances of different BrowsingInstances) but have the same renderer
-  // process.
+  // Check that A and B are in different BrowsingInstances but have the same
+  // renderer process. When default SiteInstances are enabled, A and B are
+  // both default SiteInstances of different BrowsingInstances.
   EXPECT_FALSE(a_site_instance->IsRelatedSiteInstance(b_site_instance.get()));
-  EXPECT_TRUE(a_site_instance->IsDefaultSiteInstance());
-  EXPECT_TRUE(b_site_instance->IsDefaultSiteInstance());
   EXPECT_EQ(a_site_instance->GetProcess(), b_site_instance->GetProcess());
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            a_site_instance->IsDefaultSiteInstance());
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            b_site_instance->IsDefaultSiteInstance());
 }
 
 // Different from renderer-initiated cross-site navigations, browser-initiated
@@ -5784,12 +5788,15 @@ IN_PROC_BROWSER_TEST_P(
       static_cast<SiteInstanceImpl*>(
           web_contents->GetMainFrame()->GetSiteInstance());
 
-  // Check that A and B are in different BrowsingInstances (both are in default
-  // SiteInstances of different BrowsingInstances) and renderer processes.
+  // Check that A and B are in different BrowsingInstances and renderer
+  // processes. When default SiteInstances are enabled, A and B are
+  // both default SiteInstances of different BrowsingInstances.
   EXPECT_FALSE(a_site_instance->IsRelatedSiteInstance(b_site_instance.get()));
-  EXPECT_TRUE(a_site_instance->IsDefaultSiteInstance());
-  EXPECT_TRUE(b_site_instance->IsDefaultSiteInstance());
   EXPECT_NE(a_site_instance->GetProcess(), b_site_instance->GetProcess());
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            a_site_instance->IsDefaultSiteInstance());
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            b_site_instance->IsDefaultSiteInstance());
 }
 
 // A test ContentBrowserClient implementation that enforce process-per-site mode
@@ -5844,13 +5851,14 @@ IN_PROC_BROWSER_TEST_P(
       static_cast<SiteInstanceImpl*>(
           web_contents->GetMainFrame()->GetSiteInstance());
 
-  // Check that A and B are in different BrowsingInstances (both are in default
-  // SiteInstances of different BrowsingInstances) but have the same renderer
-  // process.
+  // Check that A and B are in different BrowsingInstances but have the same
+  // renderer process.
   EXPECT_FALSE(a_site_instance->IsRelatedSiteInstance(b_site_instance.get()));
-  EXPECT_TRUE(a_site_instance->IsDefaultSiteInstance());
-  EXPECT_TRUE(b_site_instance->IsDefaultSiteInstance());
   EXPECT_EQ(b_site_instance->GetProcess(), original_process);
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            a_site_instance->IsDefaultSiteInstance());
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            b_site_instance->IsDefaultSiteInstance());
 
   // Make sure we will use process-per-site for C.
   // Note this is enforcing process-per-site for all sites, which is why we turn
@@ -5864,10 +5872,11 @@ IN_PROC_BROWSER_TEST_P(
       static_cast<SiteInstanceImpl*>(
           web_contents->GetMainFrame()->GetSiteInstance());
 
-  // Check that B and C are in different BrowsingInstances (both are in default
-  // SiteInstances of different BrowsingInstances) and renderer processes.
+  // Check that B and C are in different BrowsingInstances and renderer
+  // processes.
   EXPECT_FALSE(b_site_instance->IsRelatedSiteInstance(c_site_instance.get()));
-  EXPECT_TRUE(c_site_instance->IsDefaultSiteInstance());
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            c_site_instance->IsDefaultSiteInstance());
   EXPECT_NE(c_site_instance->GetProcess(), original_process);
   // C is using the process for C's site.
   EXPECT_EQ(c_site_instance->GetProcess(),
@@ -5885,7 +5894,8 @@ IN_PROC_BROWSER_TEST_P(
           web_contents->GetMainFrame()->GetSiteInstance());
   EXPECT_FALSE(b2_site_instance->IsRelatedSiteInstance(c_site_instance.get()));
   EXPECT_FALSE(b2_site_instance->IsRelatedSiteInstance(b_site_instance.get()));
-  EXPECT_TRUE(b2_site_instance->IsDefaultSiteInstance());
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            b2_site_instance->IsDefaultSiteInstance());
   EXPECT_NE(b2_site_instance->GetProcess(), original_process);
   // B will reuse C's process here, even though C is process-per-site, because
   // neither of them require a dedicated process.
@@ -5941,12 +5951,13 @@ IN_PROC_BROWSER_TEST_P(
       static_cast<SiteInstanceImpl*>(
           web_contents->GetMainFrame()->GetSiteInstance());
 
-  // Check that A and B are in different BrowsingInstances (both are in default
-  // SiteInstances of different BrowsingInstances) but B should use the sole
-  // process assigned to site B.
+  // Check that A and B are in different BrowsingInstances but B should use the
+  // sole process assigned to site B.
   EXPECT_FALSE(a_site_instance->IsRelatedSiteInstance(b_site_instance.get()));
-  EXPECT_TRUE(a_site_instance->IsDefaultSiteInstance());
-  EXPECT_TRUE(b_site_instance->IsDefaultSiteInstance());
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            a_site_instance->IsDefaultSiteInstance());
+  EXPECT_EQ(AreDefaultSiteInstancesEnabled(),
+            b_site_instance->IsDefaultSiteInstance());
   EXPECT_NE(b_site_instance->GetProcess(), original_process);
   EXPECT_EQ(b_site_instance->GetProcess(), process_for_b);
   EXPECT_EQ(b_site_instance->GetProcess(),
@@ -7697,6 +7708,11 @@ IN_PROC_BROWSER_TEST_P(ProactivelySwapBrowsingInstancesSameSiteTest,
   }
 
   {
+    FrameTreeNode* root = web_contents->GetFrameTree()->root();
+    RenderFrameHostImpl* child_rfh = root->child_at(0)->current_frame_host();
+    bool subframe_was_in_same_site_instance =
+        root->current_frame_host()->GetSiteInstance() ==
+        child_rfh->GetSiteInstance();
     // 4) Set up a script that will call postMessage on a cross-site iframe
     // after we commit the next navigation.
     // TODO(https://crbug.com/1110497): GetAsyncScriptExecutorCallback() must be
@@ -7722,7 +7738,8 @@ IN_PROC_BROWSER_TEST_P(ProactivelySwapBrowsingInstancesSameSiteTest,
     // have already unloaded).
     ExpectBucketCount(kActionAfterPagehideHistogramName,
                       ActionAfterPagehide::kSentPostMessage, 2);
-    if (AreAllSitesIsolatedForTesting() && !IsBackForwardCacheEnabled()) {
+
+    if (!subframe_was_in_same_site_instance && !IsBackForwardCacheEnabled()) {
       ExpectBucketCount(kActionAfterPagehideHistogramName,
                         ActionAfterPagehide::kReceivedPostMessage, 1);
     } else {
@@ -8543,12 +8560,8 @@ IN_PROC_BROWSER_TEST_P(RenderFrameHostManagerTest,
   scoped_refptr<SiteInstanceImpl> new_instance =
       web_contents->GetMainFrame()->GetSiteInstance();
   EXPECT_EQ(url1, web_contents->GetLastCommittedURL());
-  if (AreAllSitesIsolatedForTesting() || AreDefaultSiteInstancesEnabled()) {
-    EXPECT_NE(instance1, new_instance);
-    EXPECT_EQ(GURL(), new_instance->GetSiteURL());
-  } else {
-    EXPECT_EQ(instance1, new_instance);
-  }
+  EXPECT_NE(instance1, new_instance);
+  EXPECT_EQ(GURL(), new_instance->GetSiteURL());
   EXPECT_TRUE(new_instance->HasProcess());
 
   // Because url1 does not set a site URL, it should not lock the new process
diff --git a/content/browser/renderer_host/render_frame_host_manager_unittest.cc b/content/browser/renderer_host/render_frame_host_manager_unittest.cc
index 89b09163f0c133..bbbae29007d8fa 100644
--- a/content/browser/renderer_host/render_frame_host_manager_unittest.cc
+++ b/content/browser/renderer_host/render_frame_host_manager_unittest.cc
@@ -314,9 +314,9 @@ class RenderFrameHostManagerTest
     RenderViewHostImplTestHarness::SetUp();
     WebUIControllerFactory::RegisterFactory(&factory_);
 
-    if (AreDefaultSiteInstancesEnabled()) {
-      // Isolate |isolated_cross_site_url()| so we can't get a default
-      // SiteInstance for it.
+    if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
+      // Isolate |isolated_cross_site_url()|so it cannot share a process
+      // with another site.
       ChildProcessSecurityPolicyImpl::GetInstance()->AddIsolatedOrigins(
           {url::Origin::Create(isolated_cross_site_url())},
           ChildProcessSecurityPolicy::IsolatedOriginSource::TEST,
diff --git a/content/browser/security_exploit_browsertest.cc b/content/browser/security_exploit_browsertest.cc
index 065444d16da610..046084c5913b41 100644
--- a/content/browser/security_exploit_browsertest.cc
+++ b/content/browser/security_exploit_browsertest.cc
@@ -112,7 +112,7 @@ RenderFrameHostImpl* PrepareToDuplicateHosts(Shell* shell,
                                              int* target_routing_id) {
   GURL foo("http://foo.com/simple_page.html");
 
-  if (AreDefaultSiteInstancesEnabled()) {
+  if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
     // Isolate "bar.com" so we are guaranteed to get a different process
     // for navigations to this origin.
     IsolateOriginsForTesting(server, shell->web_contents(), {"bar.com"});
diff --git a/content/browser/site_instance_impl_unittest.cc b/content/browser/site_instance_impl_unittest.cc
index ec0849f9f054a0..c8fc1ddc3457fd 100644
--- a/content/browser/site_instance_impl_unittest.cc
+++ b/content/browser/site_instance_impl_unittest.cc
@@ -431,8 +431,9 @@ TEST_F(SiteInstanceTest, DefaultSiteInstanceProperties) {
   // Make sure feature list command-line options are set in a way that forces
   // default SiteInstance creation on all platforms.
   base::test::ScopedFeatureList feature_list;
-  feature_list.InitAndEnableFeature(
-      features::kProcessSharingWithDefaultSiteInstances);
+  feature_list.InitWithFeatures(
+      /* enable */ {features::kProcessSharingWithDefaultSiteInstances},
+      /* disable */ {features::kProcessSharingWithStrictSiteInstances});
   EXPECT_TRUE(base::FeatureList::IsEnabled(
       features::kProcessSharingWithDefaultSiteInstances));
   EXPECT_FALSE(base::FeatureList::IsEnabled(
@@ -1706,12 +1707,12 @@ TEST_F(SiteInstanceTest, CreateForGuest) {
       context(), UrlInfo::CreateForTesting(kGuestUrl),
       CoopCoepCrossOriginIsolatedInfo::CreateNonIsolated());
   EXPECT_FALSE(instance1->IsGuest());
-  if (AreAllSitesIsolatedForTesting()) {
+  if (AreDefaultSiteInstancesEnabled()) {
+    EXPECT_TRUE(instance1->IsDefaultSiteInstance());
+  } else {
     EXPECT_NE(kGuestUrl, instance1->GetSiteURL());
     EXPECT_EQ(GURL(std::string(kGuestScheme) + "://abc123/"),
               instance1->GetSiteURL());
-  } else {
-    EXPECT_TRUE(instance1->IsDefaultSiteInstance());
   }
 
   // Verify that a SiteInstance created with CreateForGuest() is considered
diff --git a/content/browser/web_contents/web_contents_impl_unittest.cc b/content/browser/web_contents/web_contents_impl_unittest.cc
index ed0e9d9b5b4019..f9b7b170f55b82 100644
--- a/content/browser/web_contents/web_contents_impl_unittest.cc
+++ b/content/browser/web_contents/web_contents_impl_unittest.cc
@@ -113,9 +113,9 @@ class WebContentsImplTest : public RenderViewHostImplTestHarness {
     WebUIControllerFactory::RegisterFactory(
         ContentWebUIControllerFactory::GetInstance());
 
-    if (AreDefaultSiteInstancesEnabled()) {
-      // Isolate |isolated_cross_site_url()| so we can't get a default
-      // SiteInstance for it.
+    if (IsIsolatedOriginRequiredToGuaranteeDedicatedProcess()) {
+      // Isolate |isolated_cross_site_url()| so it cannot share a process
+      // with another site.
       ChildProcessSecurityPolicyImpl::GetInstance()->AddIsolatedOrigins(
           {url::Origin::Create(isolated_cross_site_url())},
           ChildProcessSecurityPolicy::IsolatedOriginSource::TEST,
diff --git a/content/browser/web_package/web_bundle_browsertest.cc b/content/browser/web_package/web_bundle_browsertest.cc
index 75e0e3c32ca45f..b9282e96d06385 100644
--- a/content/browser/web_package/web_bundle_browsertest.cc
+++ b/content/browser/web_package/web_bundle_browsertest.cc
@@ -1370,8 +1370,8 @@ void RunIframeParentInitiatedOutOfBundleNavigationTest(
             GetLoadResultForNavigationTest(GetFirstChild(web_contents)));
 
   FrameTreeNode* iframe_node = GetFirstChild(web_contents);
-  bool is_same_process = (iframe_node->parent()->GetProcess() ==
-                          iframe_node->current_frame_host()->GetProcess());
+  bool no_proxy_to_parent =
+      iframe_node->render_manager()->GetProxyToParent() == nullptr;
 
   RunScriptAndObserveNavigation(
       "Navigate the iframe to /1-page/", web_contents,
@@ -1387,7 +1387,7 @@ void RunIframeParentInitiatedOutOfBundleNavigationTest(
   // from web bundle. To support this case we need to change
   // NavigationControllerImpl::NavigateFromFrameProxy() to correctly handle
   // the WebBundleHandleTracker.
-  EXPECT_EQ(is_same_process
+  EXPECT_EQ(no_proxy_to_parent
                 ? "/1-page/ from wbn, /1-page/script from wbn"
                 : "/1-page/ from server, /1-page/script from server",
             GetLoadResultForNavigationTest(GetFirstChild(web_contents)));
diff --git a/content/public/test/test_utils.cc b/content/public/test/test_utils.cc
index 2812abf41e6d17..f68f6e2afc816a 100644
--- a/content/public/test/test_utils.cc
+++ b/content/public/test/test_utils.cc
@@ -205,6 +205,18 @@ bool AreDefaultSiteInstancesEnabled() {
              features::kProcessSharingWithDefaultSiteInstances);
 }
 
+bool AreStrictSiteInstancesEnabled() {
+  return AreAllSitesIsolatedForTesting() ||
+         base::FeatureList::IsEnabled(
+             features::kProcessSharingWithStrictSiteInstances);
+}
+
+bool IsIsolatedOriginRequiredToGuaranteeDedicatedProcess() {
+  return AreDefaultSiteInstancesEnabled() ||
+         base::FeatureList::IsEnabled(
+             features::kProcessSharingWithStrictSiteInstances);
+}
+
 void IsolateAllSitesForTesting(base::CommandLine* command_line) {
   command_line->AppendSwitch(switches::kSitePerProcess);
 }
diff --git a/content/public/test/test_utils.h b/content/public/test/test_utils.h
index b55ad04cb7bb3e..3cd02f9a671d55 100644
--- a/content/public/test/test_utils.h
+++ b/content/public/test/test_utils.h
@@ -95,6 +95,18 @@ bool AreAllSitesIsolatedForTesting();
 // to mark expectations specific to default SiteInstances.
 bool AreDefaultSiteInstancesEnabled();
 
+// Returns true if the process model only allows a SiteInstance to contain
+// a single site.
+bool AreStrictSiteInstancesEnabled();
+
+// Returns true if a test needs to register an origin for isolation to ensure
+// that navigations, for that origin, are placed in a dedicated process. Some
+// process model modes allow sites to share a process if they are not isolated.
+// This helper indicates when such a mode is in use and indicates the test must
+// register an isolated origin to ensure the origin gets placed in its own
+// process.
+bool IsIsolatedOriginRequiredToGuaranteeDedicatedProcess();
+
 // Appends --site-per-process to the command line, enabling tests to exercise
 // site isolation and cross-process iframes. This must be called early in
 // the test; the flag will be read on the first real navigation.
diff --git a/content/test/test_web_contents.cc b/content/test/test_web_contents.cc
index efb500cf04956c..3572ec583b0021 100644
--- a/content/test/test_web_contents.cc
+++ b/content/test/test_web_contents.cc
@@ -272,7 +272,18 @@ void TestWebContents::TestDidFailLoadWithError(const GURL& url,
 }
 
 bool TestWebContents::CrossProcessNavigationPending() {
-  return GetRenderManager()->speculative_render_frame_host_ != nullptr;
+  // If we don't have a speculative RenderFrameHost then it means we did not
+  // change SiteInstances so we must be in the same process.
+  if (GetRenderManager()->speculative_render_frame_host_ == nullptr)
+    return false;
+
+  auto* current_instance =
+      GetRenderManager()->current_frame_host()->GetSiteInstance();
+  auto* speculative_instance =
+      GetRenderManager()->speculative_frame_host()->GetSiteInstance();
+  if (current_instance == speculative_instance)
+    return false;
+  return current_instance->GetProcess() != speculative_instance->GetProcess();
 }
 
 bool TestWebContents::CreateRenderViewForRenderManager(