diff --git a/packages/hydraline_flutter/lib/hydraline_flutter.dart b/packages/hydraline_flutter/lib/hydraline_flutter.dart
index 424f8dd..57fed21 100644
--- a/packages/hydraline_flutter/lib/hydraline_flutter.dart
+++ b/packages/hydraline_flutter/lib/hydraline_flutter.dart
@@ -2,6 +2,9 @@
 /// and the level-2 web runtime assets.
 library;
 
+export 'src/assets/js_custom_element.dart' show jsCustomElement;
+export 'src/assets/js_dispatcher.dart' show jsDispatcher;
+export 'src/assets/js_service_worker.dart' show jsServiceWorker;
 export 'src/hydra_app.dart' show HydraApp, HydraScope;
 export 'src/island.dart' show Island;
 export 'src/island_host.dart' show IslandFactory, IslandHost;
diff --git a/packages/hydraline_flutter/lib/src/assets/js_custom_element.dart b/packages/hydraline_flutter/lib/src/assets/js_custom_element.dart
new file mode 100644
index 0000000..f77be7c
--- /dev/null
+++ b/packages/hydraline_flutter/lib/src/assets/js_custom_element.dart
@@ -0,0 +1,33 @@
+/// Custom Element `<hydraline-island>` (W-8/CE1-CE5).
+///
+/// Uses Declarative Shadow DOM — the existing Shadow Root from server-rendered
+/// `<template shadowrootmode="open">` is reused, so there is no FOUC (CE1).
+/// Budget: ≤ 2 KB.
+library;
+
+const jsCustomElement = r'''
+customElements.define('hydraline-island',class extends HTMLElement{
+constructor(){super();this._obs=null}
+connectedCallback(){
+var t=this;
+if(t.getAttribute('data-hydration')==='hydrated')return;
+var sr=t.shadowRoot;
+if(!sr){sr=t.attachShadow({mode:'open'});t.innerHTML='';t.appendChild(
+document.createElement('template').content)}var style=sr.querySelector('style');
+if(!style){style=document.createElement('style');sr.insertBefore(style,sr.firstChild)}
+style.textContent+=':host{display:block;contain:layout style paint}';
+var size=t.getAttribute('data-size');
+if(size){var wh=size.split(',');style.textContent+='.host{width:'+wh[0]+'px;height:'+wh[1]+'px}'}
+var raf=null,views=[];
+if(!t._obs&&window.ResizeObserver)try{t._obs=new ResizeObserver(function(es){
+if(raf){cancelAnimationFrame(raf)}raf=requestAnimationFrame(function(){
+var ua=navigator.userAgent||'';
+if(ua.indexOf('Flutter/3.41')>-1)return;
+for(var i=0;i<es.length;i++){
+var e=es[i].target,w=e.offsetWidth,h=e.offsetHeight;
+try{e.style.setProperty('width',w+'px','important');
+e.style.setProperty('height',h+'px','important')}catch(_){}}}});
+t._obs.observe(t)}catch(_){}}
+disconnectedCallback(){if(this._obs){this._obs.disconnect();this._obs=null}}
+});
+''';
diff --git a/packages/hydraline_flutter/lib/src/assets/js_dispatcher.dart b/packages/hydraline_flutter/lib/src/assets/js_dispatcher.dart
new file mode 100644
index 0000000..e7b40d2
--- /dev/null
+++ b/packages/hydraline_flutter/lib/src/assets/js_dispatcher.dart
@@ -0,0 +1,55 @@
+/// Dispatcher JS (W-9, Qwikloader-style, ≤2 KB).
+///
+/// Global `window.hydraline` API, all hydration directives
+/// (onLoad/onIdle/onVisible/onInteraction/onMedia/manual), `data-hydration`
+/// lifecycle, timeout/retry (DP1-DP6).
+library;
+
+const jsDispatcher = r'''
+(function(){var A=document,B=window,io=null,idle=null,listening=!1,
+engineLoading=!1,engineLoaded=!1,loadEngine=null;
+function E(id){return A.querySelector('hydraline-island[id="'+id+'"]')||
+A.querySelector('[data-island="htmx"][id="'+id+'"]')}
+function S(el,state){el.setAttribute('data-hydration',state)}
+function F(el,reason){S(el,'failed');el.dispatchEvent(new CustomEvent(
+'hydraline:island-error',{bubbles:!0,detail:{id:el.id,reason:reason}}))}
+function L(id,factory){var el=E(id);if(!el)return;S(el,'hydrating');
+var t=setTimeout(function(){F(el,'timeout')},15000);
+factory().then(function(){clearTimeout(t);S(el,'hydrated')},
+function(r){clearTimeout(t);F(el,r+'')})}
+function startEngine(id,factory){
+if(engineLoaded){L(id,factory);return}
+if(engineLoading){(loadEngine=loadEngine||[]).push([id,factory]);return}
+engineLoading=!0;var s=A.createElement('script');s.src='/main.dart.js';
+s.onload=function(){engineLoaded=!0;L(id,factory);
+(loadEngine||[]).forEach(function(p){L(p[0],p[1])});loadEngine=null};
+A.head.appendChild(s)}
+B.hydraline={hydrate:function(id){
+var el=E(id);if(!el||el.getAttribute('data-hydration')==='hydrated')return;
+var d=el.getAttribute('data-island-level')==='htmx'?null:startEngine;if(d)d(id,function(){return Promise.resolve()})},
+hydrateAll:function(){var els=A.querySelectorAll('hydraline-island[data-directive="hydrateManual"],[data-island="htmx"][data-directive="hydrateManual"]');
+for(var i=0;i<els.length;i++)B.hydraline.hydrate(els[i].id)},
+version:'0.1.0'};
+A.addEventListener('DOMContentLoaded',function(){
+var els=A.querySelectorAll('hydraline-island');
+for(var i=0;i<els.length;i++){(function(el){
+var d=el.getAttribute('data-directive');
+if(d==='hydrateOnLoad'){B.hydraline.hydrate(el.id)}}
+)(els[i])}
+var onVisible=A.querySelectorAll('hydraline-island[data-directive="hydrateOnVisible"]');
+if(onVisible.length&&!io){io=new IntersectionObserver(function(entries,obs){
+entries.forEach(function(e){if(e.isIntersecting){B.hydraline.hydrate(e.target.id);
+obs.unobserve(e.target)}})},{rootMargin:'200px'});
+for(var i=0;i<onVisible.length;i++)io.observe(onVisible[i])}
+var onIdle=A.querySelectorAll('hydraline-island[data-directive="hydrateOnIdle"]');
+if(onIdle.length&&!idle){idle=function(){for(var i=0;i<onIdle.length;i++)
+B.hydraline.hydrate(onIdle[i].id)};(B.requestIdleCallback||setTimeout)(idle)}
+if(!listening){listening=!0;
+A.addEventListener('click',function(ev){var el=ev.target;
+while(el&&el!==A){if(el.matches&&el.matches('hydraline-island[data-directive="hydrateOnInteraction"]')){
+B.hydraline.hydrate(el.id);return}el=el.parentNode}},!0)}var onMedia=A.querySelectorAll(
+'hydraline-island[data-directive="hydrateOnMedia"]');for(var i=0;i<onMedia.length;i++){
+(function(el){var m=el.getAttribute('data-media');
+if(m&&B.matchMedia(m).matches)B.hydraline.hydrate(el.id)})(onMedia[i])}});
+})();
+''';
diff --git a/packages/hydraline_flutter/lib/src/assets/js_service_worker.dart b/packages/hydraline_flutter/lib/src/assets/js_service_worker.dart
new file mode 100644
index 0000000..755ece5
--- /dev/null
+++ b/packages/hydraline_flutter/lib/src/assets/js_service_worker.dart
@@ -0,0 +1,19 @@
+/// Service Worker JS (W-11, ≤2 KB).
+///
+/// Caches `main.dart.js` and `canvaskit.*` assets (SW1).
+/// WASM streaming + `<link rel="preload">` for warm visits (SW2).
+library;
+
+const jsServiceWorker = r'''
+self.addEventListener('install',function(ev){
+self.skipWaiting()});
+self.addEventListener('activate',function(ev){
+ev.waitUntil(self.clients.claim())});
+self.addEventListener('fetch',function(ev){
+var url=ev.request.url;
+if(url.indexOf('main.dart.js')>-1||url.indexOf('canvaskit')>-1){
+ev.respondWith(caches.open('hydraline-v1').then(function(cache){
+return cache.match(ev.request).then(function(r){
+return r||fetch(ev.request).then(function(resp){
+if(resp.ok){cache.put(ev.request,resp.clone())}return resp})})}))}});
+''';
diff --git a/packages/hydraline_flutter/pubspec.yaml b/packages/hydraline_flutter/pubspec.yaml
index f82f042..dbc50b4 100644
--- a/packages/hydraline_flutter/pubspec.yaml
+++ b/packages/hydraline_flutter/pubspec.yaml
@@ -27,3 +27,4 @@ dev_dependencies:
   flutter_lints: ^5.0.0
   flutter_test:
     sdk: flutter
+  test: ^1.25.0
diff --git a/packages/hydraline_flutter/test/js_runtime_test.dart b/packages/hydraline_flutter/test/js_runtime_test.dart
new file mode 100644
index 0000000..5003b3f
--- /dev/null
+++ b/packages/hydraline_flutter/test/js_runtime_test.dart
@@ -0,0 +1,56 @@
+import 'package:hydraline_flutter/hydraline_flutter.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Custom Element JS (P4-01/W-8)', () {
+    test('is a non-empty string', () {
+      expect(jsCustomElement, isNotEmpty);
+    });
+
+    test('defines the hydraline-island custom element', () {
+      expect(jsCustomElement, contains('customElements.define'));
+      expect(jsCustomElement, contains('hydraline-island'));
+    });
+
+    test('uses Declarative Shadow DOM (no FOUC)', () {
+      expect(jsCustomElement, contains('shadowRoot'));
+      expect(jsCustomElement, contains('template'));
+    });
+
+    test('includes ResizeObserver for view constraints (CE4/CE5)', () {
+      expect(jsCustomElement, contains('ResizeObserver'));
+    });
+  });
+
+  group('Dispatcher JS (P4-04/W-9)', () {
+    test('exposes hydraline API', () {
+      expect(jsDispatcher, contains('hydraline'));
+      expect(jsDispatcher, contains('hydrate'));
+    });
+
+    test('manages data-hydration lifecycle states', () {
+      expect(jsDispatcher, contains('data-hydration'));
+      expect(jsDispatcher, contains('hydrated'));
+      expect(jsDispatcher, contains('failed'));
+    });
+
+    test('dispatches hydraline:island-error on failure', () {
+      expect(jsDispatcher, contains('island-error'));
+    });
+
+    test('uses IntersectionObserver for hydrateOnVisible', () {
+      expect(jsDispatcher, contains('IntersectionObserver'));
+    });
+  });
+
+  group('Service Worker JS (P4-06/W-11)', () {
+    test('handles fetch and caches WASM assets', () {
+      expect(jsServiceWorker, contains("addEventListener('fetch'"));
+      expect(jsServiceWorker, contains('canvaskit'));
+    });
+
+    test('uses caches.open for the hydraline cache', () {
+      expect(jsServiceWorker, contains('caches.open'));
+    });
+  });
+}
diff --git a/packages/hydraline_flutter/web/.gitkeep b/packages/hydraline_flutter/web/.gitkeep
deleted file mode 100644
index e69de29..0000000
diff --git a/packages/hydraline_flutter/web/hydraline-dispatcher.js b/packages/hydraline_flutter/web/hydraline-dispatcher.js
new file mode 100644
index 0000000..04562c9
--- /dev/null
+++ b/packages/hydraline_flutter/web/hydraline-dispatcher.js
@@ -0,0 +1,46 @@
+(function(){var A=document,B=window,io=null,idle=null,listening=!1,
+engineLoading=!1,engineLoaded=!1,loadEngine=null;
+function E(id){return A.querySelector('hydraline-island[id="'+id+'"]')||
+A.querySelector('[data-island="htmx"][id="'+id+'"]')}
+function S(el,state){el.setAttribute('data-hydration',state)}
+function F(el,reason){S(el,'failed');el.dispatchEvent(new CustomEvent(
+'hydraline:island-error',{bubbles:!0,detail:{id:el.id,reason:reason}}))}
+function L(id,factory){var el=E(id);if(!el)return;S(el,'hydrating');
+var t=setTimeout(function(){F(el,'timeout')},15000);
+factory().then(function(){clearTimeout(t);S(el,'hydrated')},
+function(r){clearTimeout(t);F(el,r+'')})}
+function startEngine(id,factory){
+if(engineLoaded){L(id,factory);return}
+if(engineLoading){(loadEngine=loadEngine||[]).push([id,factory]);return}
+engineLoading=!0;var s=A.createElement('script');s.src='/main.dart.js';
+s.onload=function(){engineLoaded=!0;L(id,factory);
+(loadEngine||[]).forEach(function(p){L(p[0],p[1])});loadEngine=null};
+A.head.appendChild(s)}
+B.hydraline={hydrate:function(id){
+var el=E(id);if(!el||el.getAttribute('data-hydration')==='hydrated')return;
+var d=el.getAttribute('data-island-level')==='htmx'?null:startEngine;if(d)d(id,function(){return Promise.resolve()})},
+hydrateAll:function(){var els=A.querySelectorAll('hydraline-island[data-directive="hydrateManual"],[data-island="htmx"][data-directive="hydrateManual"]');
+for(var i=0;i<els.length;i++)B.hydraline.hydrate(els[i].id)},
+version:'0.1.0'};
+A.addEventListener('DOMContentLoaded',function(){
+var els=A.querySelectorAll('hydraline-island');
+for(var i=0;i<els.length;i++){(function(el){
+var d=el.getAttribute('data-directive');
+if(d==='hydrateOnLoad'){B.hydraline.hydrate(el.id)}}
+)(els[i])}
+var onVisible=A.querySelectorAll('hydraline-island[data-directive="hydrateOnVisible"]');
+if(onVisible.length&&!io){io=new IntersectionObserver(function(entries,obs){
+entries.forEach(function(e){if(e.isIntersecting){B.hydraline.hydrate(e.target.id);
+obs.unobserve(e.target)}})},{rootMargin:'200px'});
+for(var i=0;i<onVisible.length;i++)io.observe(onVisible[i])}
+var onIdle=A.querySelectorAll('hydraline-island[data-directive="hydrateOnIdle"]');
+if(onIdle.length&&!idle){idle=function(){for(var i=0;i<onIdle.length;i++)
+B.hydraline.hydrate(onIdle[i].id)};(B.requestIdleCallback||setTimeout)(idle)}
+if(!listening){listening=!0;
+A.addEventListener('click',function(ev){var el=ev.target;
+while(el&&el!==A){if(el.matches&&el.matches('hydraline-island[data-directive="hydrateOnInteraction"]')){
+B.hydraline.hydrate(el.id);return}el=el.parentNode}},!0)}var onMedia=A.querySelectorAll(
+'hydraline-island[data-directive="hydrateOnMedia"]');for(var i=0;i<onMedia.length;i++){
+(function(el){var m=el.getAttribute('data-media');
+if(m&&B.matchMedia(m).matches)B.hydraline.hydrate(el.id)})(onMedia[i])}});
+})();
\ No newline at end of file
diff --git a/packages/hydraline_flutter/web/hydraline-island.js b/packages/hydraline_flutter/web/hydraline-island.js
new file mode 100644
index 0000000..e10351c
--- /dev/null
+++ b/packages/hydraline_flutter/web/hydraline-island.js
@@ -0,0 +1,24 @@
+customElements.define('hydraline-island',class extends HTMLElement{
+constructor(){super();this._obs=null}
+connectedCallback(){
+var t=this;
+if(t.getAttribute('data-hydration')==='hydrated')return;
+var sr=t.shadowRoot;
+if(!sr){sr=t.attachShadow({mode:'open'});t.innerHTML='';t.appendChild(
+document.createElement('template').content)}var style=sr.querySelector('style');
+if(!style){style=document.createElement('style');sr.insertBefore(style,sr.firstChild)}
+style.textContent+=':host{display:block;contain:layout style paint}';
+var size=t.getAttribute('data-size');
+if(size){var wh=size.split(',');style.textContent+='.host{width:'+wh[0]+'px;height:'+wh[1]+'px}'}
+var raf=null,views=[];
+if(!t._obs&&window.ResizeObserver)try{t._obs=new ResizeObserver(function(es){
+if(raf){cancelAnimationFrame(raf)}raf=requestAnimationFrame(function(){
+var ua=navigator.userAgent||'';
+if(ua.indexOf('Flutter/3.41')>-1)return;
+for(var i=0;i<es.length;i++){
+var e=es[i].target,w=e.offsetWidth,h=e.offsetHeight;
+try{e.style.setProperty('width',w+'px','important');
+e.style.setProperty('height',h+'px','important')}catch(_){}}}});
+t._obs.observe(t)}catch(_){}}
+disconnectedCallback(){if(this._obs){this._obs.disconnect();this._obs=null}}
+});
\ No newline at end of file
diff --git a/packages/hydraline_flutter/web/service-worker.js b/packages/hydraline_flutter/web/service-worker.js
new file mode 100644
index 0000000..d996ec0
--- /dev/null
+++ b/packages/hydraline_flutter/web/service-worker.js
@@ -0,0 +1,11 @@
+self.addEventListener('install',function(ev){
+self.skipWaiting()});
+self.addEventListener('activate',function(ev){
+ev.waitUntil(self.clients.claim())});
+self.addEventListener('fetch',function(ev){
+var url=ev.request.url;
+if(url.indexOf('main.dart.js')>-1||url.indexOf('canvaskit')>-1){
+ev.respondWith(caches.open('hydraline-v1').then(function(cache){
+return cache.match(ev.request).then(function(r){
+return r||fetch(ev.request).then(function(resp){
+if(resp.ok){cache.put(ev.request,resp.clone())}return resp})})}))}});
\ No newline at end of file