diff --git a/play.html b/play.html
index 4c16fac..04ea13b 100644
--- a/play.html
+++ b/play.html
@@ -199,20 +199,25 @@
 
       #play-stage-wrapper {
         flex: 1 1 auto;
+        width: 100%;
+        height: 100%;
         min-width: 0;
         min-height: 0;
         display: flex;
         align-items: center;
         justify-content: center;
-        background: radial-gradient(circle at center, #14110d 0%, #040404 75%);
+        background: #000;
         position: relative;
         overflow: hidden;
       }
 
       #play-stage {
         position: relative;
-        width: 800px;
-        height: 600px;
+        aspect-ratio: 4 / 3;
+        width: min(100%, calc((100vh - 36px) * 4 / 3));
+        height: min(100%, calc(100vw * 3 / 4));
+        max-width: 100%;
+        max-height: 100%;
         background: #000;
         box-shadow: 0 0 42px rgba(0, 0, 0, 0.95), 0 0 0 1px #2e251b;
         overflow: hidden;
@@ -243,6 +248,7 @@
         display: block;
         background: #080706;
         z-index: 5;
+        image-rendering: pixelated;
       }
 
       #d2-hud-canvas {
diff --git a/src/client/main.ts b/src/client/main.ts
index 5a13693..da526ba 100644
--- a/src/client/main.ts
+++ b/src/client/main.ts
@@ -123,6 +123,14 @@ function drawEscapeMenuOverlay(
   height: number,
 ): void {
   ctx.save()
+  const canvasW = ctx.canvas?.width ?? width
+  const canvasH = ctx.canvas?.height ?? height
+  const scaleX = width > 0 && canvasW > 0 ? canvasW / width : 1
+  const scaleY = height > 0 && canvasH > 0 ? canvasH / height : 1
+  if (scaleX !== 1 || scaleY !== 1) {
+    ctx.scale(scaleX, scaleY)
+  }
+
   ctx.fillStyle = 'rgba(0, 0, 0, 0.52)'
   ctx.fillRect(0, 0, width, height)
 
@@ -153,6 +161,7 @@ async function bootPlayApp(): Promise<PlayAppController> {
 
   const toolbarContainer = document.querySelector<HTMLElement>('#d2-toolbar-container')
   const inspectorContainer = document.querySelector<HTMLElement>('#d2-inspector-container')
+  const stageWrapperEl = document.querySelector<HTMLElement>('#play-stage-wrapper')
   const stageEl = document.querySelector<HTMLElement>('#play-stage')
   const bnetShell = document.querySelector<HTMLElement>('#bnet-shell')
   const bnetCanvas = document.querySelector<HTMLCanvasElement>('#bnet-canvas')
@@ -172,6 +181,20 @@ async function bootPlayApp(): Promise<PlayAppController> {
 
   let sessionRef: OnlineSession | null = null
 
+  const syncStageViewportFit = (
+    profileOrId: ViewportProfile | typeof initialSettings.viewport = settingsStore.get().viewport,
+  ): ViewportProfile => {
+    const profile = applyViewportProfileToDom(profileOrId, {
+      containerEl: stageWrapperEl ?? stageEl.parentElement,
+      stageEl,
+      glCanvas: viewCanvas,
+      hudCanvas,
+    })
+    sessionRef?.hudManager.resize(profile.width, profile.height)
+    sessionRef?.camera.resize(profile)
+    return profile
+  }
+
   const toolbar = new Toolbar({
     store: settingsStore,
     container: toolbarContainer,
@@ -181,19 +204,14 @@ async function bootPlayApp(): Promise<PlayAppController> {
         sessionRef?.hudManager.setLang(lang)
       },
       onViewportChange: (profile: ViewportProfile) => {
-        applyViewportProfileToDom(profile, {
-          stageEl,
-          glCanvas: viewCanvas,
-          hudCanvas,
-        })
-        sessionRef?.hudManager.resize(profile.width, profile.height)
-        sessionRef?.camera.resize(profile)
+        syncStageViewportFit(profile)
       },
       onAutomapRevealChange: (mode) => {
         sessionRef?.automapView.setMode(mode)
       },
       onInspectorToggle: (open) => {
         sessionRef?.packetInspector.setOpen(open)
+        syncStageViewportFit()
       },
       onServerHostReconnect: (newHost) => {
         void sessionRef?.disconnect(`Reconnecting to ${newHost}`)
@@ -375,26 +393,23 @@ async function bootPlayApp(): Promise<PlayAppController> {
     toolbar.setAssetCacheUsageBytes(updatedStatus.usageBytes)
   })
 
-  // Apply initial viewport dimensions (800×600 or 1024×768)
-  const initialProfile = getViewportProfile(initialSettings.viewport)
-  applyViewportProfileToDom(initialProfile, {
-    stageEl,
-    glCanvas: viewCanvas,
-    hudCanvas,
+  // Apply initial viewport dimensions (800×600 or 1024×768) stretched to fit browser
+  syncStageViewportFit(initialSettings.viewport)
+
+  window.addEventListener('resize', () => {
+    syncStageViewportFit()
   })
-  session.hudManager.resize(initialProfile.width, initialProfile.height)
-  session.camera.resize(initialProfile)
+
+  if (typeof ResizeObserver !== 'undefined' && stageWrapperEl) {
+    const stageResizeObserver = new ResizeObserver(() => {
+      syncStageViewportFit()
+    })
+    stageResizeObserver.observe(stageWrapperEl)
+  }
 
   settingsStore.subscribe((next, prev) => {
     if (next.viewport !== prev.viewport) {
-      const profile = getViewportProfile(next.viewport)
-      applyViewportProfileToDom(profile, {
-        stageEl,
-        glCanvas: viewCanvas,
-        hudCanvas,
-      })
-      session.hudManager.resize(profile.width, profile.height)
-      session.camera.resize(profile)
+      syncStageViewportFit(next.viewport)
     }
     if (next.lang !== prev.lang) {
       setLang(next.lang)
@@ -405,6 +420,7 @@ async function bootPlayApp(): Promise<PlayAppController> {
     }
     if (next.inspectorOpen !== prev.inspectorOpen) {
       inspector.setOpen(next.inspectorOpen)
+      syncStageViewportFit(next.viewport)
     }
   })
 
diff --git a/src/client/view/viewport-profile.ts b/src/client/view/viewport-profile.ts
index 454039a..bdd0902 100644
--- a/src/client/view/viewport-profile.ts
+++ b/src/client/view/viewport-profile.ts
@@ -232,8 +232,83 @@ export function screenToSubTile(
   }
 }
 
+export interface ViewportFitDimensions {
+  readonly logicalWidth: number
+  readonly logicalHeight: number
+  readonly containerWidth: number
+  readonly containerHeight: number
+  readonly scale: number
+  readonly cssWidth: number
+  readonly cssHeight: number
+  readonly letterboxLeft: number
+  readonly letterboxTop: number
+}
+
+/**
+ * Compute the largest aspect-ratio-preserving (`4:3`) CSS box (`cssWidth × cssHeight`)
+ * that fits inside `(containerWidth × containerHeight)`, retaining side black bars (pillarboxing)
+ * on widescreen containers or top/bottom black bars (letterboxing) on narrow containers.
+ */
+export function computeViewportFitSize(
+  profileOrId: ViewportProfile | ViewportResolution,
+  containerWidth: number,
+  containerHeight: number,
+): ViewportFitDimensions {
+  const profile = getViewportProfile(profileOrId)
+  const validContainer =
+    Number.isFinite(containerWidth) &&
+    Number.isFinite(containerHeight) &&
+    containerWidth > 0 &&
+    containerHeight > 0
+
+  if (!validContainer) {
+    return {
+      logicalWidth: profile.width,
+      logicalHeight: profile.height,
+      containerWidth: profile.width,
+      containerHeight: profile.height,
+      scale: 1,
+      cssWidth: profile.width,
+      cssHeight: profile.height,
+      letterboxLeft: 0,
+      letterboxTop: 0,
+    }
+  }
+
+  const scale = Math.min(containerWidth / profile.width, containerHeight / profile.height)
+  const cssWidth = Math.max(1, Math.round(profile.width * scale))
+  const cssHeight = Math.max(1, Math.round(profile.height * scale))
+  const letterboxLeft = Math.max(0, Math.floor((containerWidth - cssWidth) / 2))
+  const letterboxTop = Math.max(0, Math.floor((containerHeight - cssHeight) / 2))
+
+  return {
+    logicalWidth: profile.width,
+    logicalHeight: profile.height,
+    containerWidth,
+    containerHeight,
+    scale,
+    cssWidth,
+    cssHeight,
+    letterboxLeft,
+    letterboxTop,
+  }
+}
+
+export interface ViewportContainerLike {
+  readonly clientWidth: number
+  readonly clientHeight: number
+}
+
 export interface ViewportDomTargets {
-  readonly stageEl?: { style: { width: string; height: string } } | null | undefined
+  readonly containerEl?: ViewportContainerLike | null | undefined
+  readonly stageEl?:
+    | {
+        style: { width: string; height: string }
+        parentElement?: ViewportContainerLike | null
+        setAttribute?: (name: string, value: string) => void
+      }
+    | null
+    | undefined
   readonly glCanvas?: { width: number; height: number; style?: { width: string; height: string } } | null | undefined
   readonly hudCanvas?: { width: number; height: number; style?: { width: string; height: string } } | null | undefined
   readonly onResize?: ((profile: ViewportProfile) => void) | undefined
@@ -241,32 +316,48 @@ export interface ViewportDomTargets {
 
 /**
  * Resize the stage container, WebGL2 canvas, and 2D HUD overlay canvas in lockstep.
+ * When a container (`containerEl` or `stageEl.parentElement`) with positive dimensions is available,
+ * stretches the stage CSS box to fit the container while preserving the `4:3` aspect ratio.
  */
 export function applyViewportProfileToDom(
   profileOrId: ViewportProfile | ViewportResolution,
   targets: ViewportDomTargets,
 ): ViewportProfile {
   const profile = getViewportProfile(profileOrId)
+  const container = targets.containerEl ?? targets.stageEl?.parentElement ?? null
+  const hasContainerBounds =
+    container !== null &&
+    Number.isFinite(container.clientWidth) &&
+    Number.isFinite(container.clientHeight) &&
+    container.clientWidth > 0 &&
+    container.clientHeight > 0
+
+  const fit = hasContainerBounds
+    ? computeViewportFitSize(profile, container.clientWidth, container.clientHeight)
+    : null
+
   if (targets.stageEl) {
-    targets.stageEl.style.width = `${profile.width}px`
-    targets.stageEl.style.height = `${profile.height}px`
+    targets.stageEl.style.width = fit ? `${fit.cssWidth}px` : `${profile.width}px`
+    targets.stageEl.style.height = fit ? `${fit.cssHeight}px` : `${profile.height}px`
+    targets.stageEl.setAttribute?.('data-viewport', profile.id)
   }
   if (targets.glCanvas) {
     targets.glCanvas.width = profile.width
     targets.glCanvas.height = profile.height
     if (targets.glCanvas.style) {
-      targets.glCanvas.style.width = `${profile.width}px`
-      targets.glCanvas.style.height = `${profile.height}px`
+      targets.glCanvas.style.width = fit ? '100%' : `${profile.width}px`
+      targets.glCanvas.style.height = fit ? '100%' : `${profile.height}px`
     }
   }
   if (targets.hudCanvas) {
     targets.hudCanvas.width = profile.width
     targets.hudCanvas.height = profile.height
     if (targets.hudCanvas.style) {
-      targets.hudCanvas.style.width = `${profile.width}px`
-      targets.hudCanvas.style.height = `${profile.height}px`
+      targets.hudCanvas.style.width = fit ? '100%' : `${profile.width}px`
+      targets.hudCanvas.style.height = fit ? '100%' : `${profile.height}px`
     }
   }
   targets.onResize?.(profile)
   return profile
 }
+
diff --git a/tests/client/settings-toolbar-view.test.ts b/tests/client/settings-toolbar-view.test.ts
index 2bcf569..f4ad88a 100644
--- a/tests/client/settings-toolbar-view.test.ts
+++ b/tests/client/settings-toolbar-view.test.ts
@@ -41,6 +41,7 @@ import {
 import {
   applyViewportProfileToDom,
   computeViewportAnchors,
+  computeViewportFitSize,
   getViewportProfile,
   screenToSubTile,
   screenToWorld,
@@ -363,6 +364,51 @@ describe('Dual Viewport Profiles, HudManager Anchoring & Coordinate Transforms (
     expect(glCanvas.width).toBe(1024)
     expect(hudCanvas.height).toBe(768)
   })
+
+  it('stretches both 800x600 and 1024x768 to fit the browser container while retaining 4:3 aspect ratio and side black bars', () => {
+    // Widescreen 1920x896 wrapper (1920x932 browser minus 36px toolbar):
+    const fit800 = computeViewportFitSize('800x600', 1920, 896)
+    expect(fit800.cssHeight).toBe(896)
+    expect(fit800.cssWidth).toBe(Math.round(896 * (4 / 3)))
+    expect(fit800.letterboxLeft).toBeGreaterThan(0)
+    expect(fit800.letterboxTop).toBe(0)
+
+    const fit1024 = computeViewportFitSize('1024x768', 1920, 896)
+    expect(fit1024.cssHeight).toBe(896)
+    expect(fit1024.cssWidth).toBe(fit800.cssWidth)
+    expect(fit1024.letterboxLeft).toBe(fit800.letterboxLeft)
+
+    // Narrow/portrait 800x1000 container:
+    const fitNarrow = computeViewportFitSize('1024x768', 800, 1000)
+    expect(fitNarrow.cssWidth).toBe(800)
+    expect(fitNarrow.cssHeight).toBe(600)
+    expect(fitNarrow.letterboxLeft).toBe(0)
+    expect(fitNarrow.letterboxTop).toBe(200)
+
+    // DOM application with containerEl stretches stage CSS to fit while keeping internal canvas resolution exact
+    const containerEl = { clientWidth: 1920, clientHeight: 1080 }
+    const stageEl = { style: { width: '', height: '' } }
+    const glCanvas = { width: 0, height: 0, style: { width: '', height: '' } }
+    const hudCanvas = { width: 0, height: 0, style: { width: '', height: '' } }
+
+    applyViewportProfileToDom('800x600', { containerEl, stageEl, glCanvas, hudCanvas })
+    expect(stageEl.style.width).toBe('1440px')
+    expect(stageEl.style.height).toBe('1080px')
+    expect(glCanvas.width).toBe(800)
+    expect(glCanvas.height).toBe(600)
+    expect(glCanvas.style.width).toBe('100%')
+    expect(glCanvas.style.height).toBe('100%')
+    expect(hudCanvas.style.width).toBe('100%')
+    expect(hudCanvas.style.height).toBe('100%')
+
+    applyViewportProfileToDom('1024x768', { containerEl, stageEl, glCanvas, hudCanvas })
+    expect(stageEl.style.width).toBe('1440px')
+    expect(stageEl.style.height).toBe('1080px')
+    expect(glCanvas.width).toBe(1024)
+    expect(glCanvas.height).toBe(768)
+    expect(glCanvas.style.width).toBe('100%')
+    expect(glCanvas.style.height).toBe('100%')
+  })
 })
 
 describe('SceneSource, UnitDrawables, Missiles/Overlays & LightingInputs (F15, F17)', () => {