diff --git a/moli-renderer-v8/src/runtime/owner_maintenance.rs b/moli-renderer-v8/src/runtime/owner_maintenance.rs
index 35eb31ade7..e64d384937 100644
--- a/moli-renderer-v8/src/runtime/owner_maintenance.rs
+++ b/moli-renderer-v8/src/runtime/owner_maintenance.rs
@@ -29,9 +29,10 @@ const ACTIVE_PAGE_MODERATE_MEMORY_PRESSURE_PERIOD: Duration = Duration::from_sec
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub(super) enum RendererOwnerMaintenanceAction {
-    /// Ask V8 to perform its moderate-memory-pressure maintenance for the
-    /// isolate currently attached to this Page.
-    ModerateMemoryPressure,
+    /// Ask V8 to perform memory-pressure maintenance for the isolate currently
+    /// attached to this Page. The isolate selects moderate or critical
+    /// pressure from its current heap occupancy.
+    MemoryPressure,
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -102,7 +103,7 @@ impl RendererPageOwnerMaintenanceResidence {
         };
         Some(RendererOwnerMaintenanceTask {
             token,
-            action: RendererOwnerMaintenanceAction::ModerateMemoryPressure,
+            action: RendererOwnerMaintenanceAction::MemoryPressure,
             scheduled_for: deadline,
         })
     }
@@ -143,11 +144,11 @@ pub(super) async fn execute_owner_maintenance_task_on_local_lane(
     run_entry_on_bound_owner_local_store_local_task(local_executor, entry, move |entry| {
         Box::pin(async move {
             match task.action() {
-                RendererOwnerMaintenanceAction::ModerateMemoryPressure => entry
+                RendererOwnerMaintenanceAction::MemoryPressure => entry
                     .page_vm_mut()
                     .vm_mut()
                     .renderer_document_isolate_ops()
-                    .notify_renderer_document_isolate_moderate_memory_pressure(),
+                    .notify_renderer_document_isolate_memory_pressure(),
             }
         })
     })
diff --git a/moli-renderer-v8/src/script_vm.rs b/moli-renderer-v8/src/script_vm.rs
index 2fc4f748f1..35e7139cbc 100644
--- a/moli-renderer-v8/src/script_vm.rs
+++ b/moli-renderer-v8/src/script_vm.rs
@@ -53,6 +53,17 @@ use url::Url;
 
 pub(crate) type ScriptVmBootstrapError = Box<(anyhow::Error, DomHost)>;
 
+fn renderer_document_isolate_critical_pressure_required(
+    used_heap_size: usize,
+    heap_size_limit: usize,
+) -> bool {
+    if heap_size_limit == 0 {
+        return false;
+    }
+    let critical_threshold = heap_size_limit / 3 + usize::from(!heap_size_limit.is_multiple_of(3));
+    used_heap_size >= critical_threshold
+}
+
 #[cfg(test)]
 fn expect_ready_child_frame_owner_source_future_for_test<F>(future: F) -> F::Output
 where
@@ -6742,10 +6753,24 @@ impl ScriptVm {
             })
     }
 
-    fn notify_renderer_document_isolate_moderate_memory_pressure(&mut self) -> Result<()> {
+    fn notify_renderer_document_isolate_memory_pressure(&mut self) -> Result<()> {
         self.renderer_document_isolate
             .with_entered_renderer_document_isolate(|isolate| {
-                isolate.memory_pressure_notification(v8::MemoryPressureLevel::Moderate);
+                let stats = isolate.get_heap_statistics();
+                if renderer_document_isolate_critical_pressure_required(
+                    stats.used_heap_size(),
+                    stats.heap_size_limit(),
+                ) {
+                    // One renderer process can host several Page isolates. A
+                    // target with a rapidly replaced child realm must not hit
+                    // V8's process-fatal heap limit before its owner can close
+                    // that target. Escalate the existing periodic maintenance
+                    // once this isolate consumes a third of its own limit.
+                    isolate.memory_pressure_notification(v8::MemoryPressureLevel::Critical);
+                    isolate.low_memory_notification();
+                } else {
+                    isolate.memory_pressure_notification(v8::MemoryPressureLevel::Moderate);
+                }
                 Ok(())
             })
     }
@@ -6821,11 +6846,8 @@ impl ScriptVmRendererDocumentIsolateOps<'_> {
         self.vm.collect_renderer_document_isolate_garbage()
     }
 
-    pub(super) fn notify_renderer_document_isolate_moderate_memory_pressure(
-        &mut self,
-    ) -> Result<()> {
-        self.vm
-            .notify_renderer_document_isolate_moderate_memory_pressure()
+    pub(super) fn notify_renderer_document_isolate_memory_pressure(&mut self) -> Result<()> {
+        self.vm.notify_renderer_document_isolate_memory_pressure()
     }
 }
 
diff --git a/moli-renderer-v8/src/script_vm/tests/mod.rs b/moli-renderer-v8/src/script_vm/tests/mod.rs
index e3ba66fd47..940d8c604c 100644
--- a/moli-renderer-v8/src/script_vm/tests/mod.rs
+++ b/moli-renderer-v8/src/script_vm/tests/mod.rs
@@ -5,7 +5,7 @@ use super::{
     PostParseProcessingAction, PostParseRuntimeDriverStep, PostParseStageBoundary,
     PostParseTaskCompletion, PostParseTaskExecutionToken, PostParseTaskInvalidationPolicy,
     ReadyPostParseAction, ScriptVm, ScriptVmDefaultWorldBootstrap, StandaloneScriptVmHarness,
-    select_post_parse_driver_step,
+    renderer_document_isolate_critical_pressure_required, select_post_parse_driver_step,
 };
 use crate::document_runtime::{
     CurrentScriptContextSpec, DeferredPageTask, DeferredPageTaskLane, DeferredPageTaskState,
@@ -5193,6 +5193,20 @@ fn dom_wrapper_expando_survives_renderer_document_isolate_garbage_collection() {
     assert_eq!(retained, "retained");
 }
 
+#[test]
+fn renderer_document_isolate_memory_pressure_escalates_at_one_third_of_heap_limit() {
+    assert!(!renderer_document_isolate_critical_pressure_required(0, 0));
+    assert!(!renderer_document_isolate_critical_pressure_required(
+        33, 100
+    ));
+    assert!(renderer_document_isolate_critical_pressure_required(
+        34, 100
+    ));
+    assert!(renderer_document_isolate_critical_pressure_required(
+        100, 100
+    ));
+}
+
 #[test]
 fn context_wrapper_cache_is_cleared_on_script_vm_teardown() {
     let mut vm = new_parsed_test_vm(