diff --git a/CHANGELOG.md b/CHANGELOG.md
index 59532d4..d701dc9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,16 @@
 
 ## Unreleased
 
+- Design scoped effects, strict CSP nonce support, and stronger host compatibility (closes #13).
+  - Add `effectScope` prop to `AccessibilityPlugin` supporting CSS selector strings, HTMLElements, or React ref objects to restrict built-in effects strictly to designated host containers.
+  - Apply CSS filters (grayscale, invert, saturation) directly to the scoped container, completely protecting external fixed headers, navbars, and banners from filter containing-block/stacking-context positioning issues.
+  - Add open Shadow DOM traversal (`mode === "open"`), allowing accessibility enhancements to reach Web Component internals.
+  - Add `nonce` prop to `AccessibilityPlugin` and propagate nonce attributes to dynamically injected style elements (`arap-focus-highlight-style`, `arap-reduce-motion-style`) for strict CSP compliance.
+  - Export `resolveEffectScope`, `type EffectScopeTarget`, and `type ApplyPageOptions` from `src/index.ts`.
+  - Add `docs/SCOPED_EFFECTS.md` detailing architecture, container targeting, Shadow DOM traversal, iframe boundaries, and CSP nonces.
+  - Update `docs/API.md`, `docs/ROADMAP.md`, and `docs/PROJECT_STATUS.md`.
+  - Add 11 unit and integration tests in `tests/scopedEffects.test.tsx` and real browser Playwright test in `tests/browser/widget.spec.ts`.
+
 - Profile large documents, add incremental mutation batching, late web font metric refresh, and root attribute observation (closes #12).
   - Increase eligible element capacity cap from 4,000 to 8,000 nodes (`MAX_HOST_ELEMENTS = 8000`).
   - Add incremental `childList` mutation batching in `applyPageSession`, preserving existing styled nodes and styling newly added candidate subtrees without flash of unstyled content (FOUC) or full-document teardowns.
diff --git a/docs/API.md b/docs/API.md
index af2cd2c..a39378a 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -163,6 +163,22 @@ React hook returning reactive diagnostics reports.
 - Type: `(event: AccessibilityFeatureEvent) => void`
 - Optional local lifecycle callback triggered on user interactions (toggles, steppers, resets, profiles, presets, panel visibility, placement). Disabled by default.
 
+### `AccessibilityPluginProps.effectScope` (Issue #13)
+
+- Type: `string | HTMLElement | { current: HTMLElement | null } | null`
+- Optional scoped container target for built-in effects. Accepts a CSS selector string (e.g. `"#main-content"`), an HTMLElement, or a React ref object.
+- Defaults to `document.body` (host-wide) when omitted.
+- When specified:
+  - Built-in typography and display effects are restricted to the target container and its descendants.
+  - CSS filters (grayscale, invert, saturation) apply directly to the scoped container, protecting external fixed elements (such as host headers, navbars, and banners) from filter containing-block/stacking-context bugs.
+  - See [docs/SCOPED_EFFECTS.md](SCOPED_EFFECTS.md).
+
+### `AccessibilityPluginProps.nonce` (Issue #13)
+
+- Type: `string`
+- Optional Content Security Policy (CSP) nonce attribute passed to all dynamically injected `<style>` elements (`arap-focus-highlight-style`, `arap-reduce-motion-style`).
+- Enables zero CSP violations in environments with strict `style-src 'nonce-...'` directives.
+
 ### `AccessibilityFeatureEvent` Types
 
 ```ts
diff --git a/docs/PROJECT_STATUS.md b/docs/PROJECT_STATUS.md
index c825c5b..1354315 100644
--- a/docs/PROJECT_STATUS.md
+++ b/docs/PROJECT_STATUS.md
@@ -13,16 +13,16 @@ and the release proposal and version recommendation are documented in [RELEASE_P
 
 ## Status categories
 
-| Category                   | Current evidence                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
-| -------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
-| Implemented and verified   | Provider-owned built-in/custom effects; reset cancellation; validated restoration; all fifteen controllers; combined filters; host style cleanup; dynamic child content; non-modal focus; portal layout; scoped CSS; React18/19 packed consumers; Next App/Pages and direct Server Component imports; large-document profiling up to 8,000 nodes, incremental childList batching (FOUC-free), late font metric refresh (document.fonts), and root attribute observation (class, data-theme, dir, lang) |
-| Implemented but unverified | Broader React19 patch versions, physical devices, assistive technologies and browser versions outside the executed matrix                                                                                                                                                                                                                                                                                                                                                                              |
-| Partially implemented      | Broad accessibility acceptance: automated keyboard/axe/reflow checks passed, real screen-reader/physical-device/actual zoom review remains. Host-wide integration: known filter/CSP/Shadow DOM limits remain.                                                                                                                                                                                                                                                                                          |
-| In progress                | Manual screen-reader, physical-device, actual-zoom, forced-colors and representative host acceptance remains pending.                                                                                                                                                                                                                                                                                                                                                                                  |
-| Planned                    | Explicit effect scopes (Issue #13), CSP renderer, Shadow DOM strategy, and richer localization                                                                                                                                                                                                                                                                                                                                                                                                         |
-| Blocked                    | Local macOS Firefox launch only (Linux coverage passed); stable human acceptance remains pending                                                                                                                                                                                                                                                                                                                                                                                                       |
-| Deferred                   | Multi-provider arbitration and live cross-tab sync                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
-| Completed                  | Investigation/baseline, local P0 stabilization, feature regression suite, packed consumer/build verification, documentation, local example verification and full hosted CI meet their stated local acceptance criteria                                                                                                                                                                                                                                                                                 |
+| Category                   | Current evidence                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
+| -------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| Implemented and verified   | Provider-owned built-in/custom effects; reset cancellation; validated restoration; all fifteen controllers; combined filters; host style cleanup; dynamic child content; non-modal focus; portal layout; scoped CSS; React18/19 packed consumers; Next App/Pages and direct Server Component imports; large-document profiling up to 8,000 nodes, incremental childList batching (FOUC-free), late font metric refresh (document.fonts), and root attribute observation (class, data-theme, dir, lang); scoped effect targeting (effectScope), strict CSP nonce propagation (nonce), open Shadow DOM traversal, and fixed-descendant filter isolation (Issue #13) |
+| Implemented but unverified | Broader React19 patch versions, physical devices, assistive technologies and browser versions outside the executed matrix                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
+| Partially implemented      | Broad accessibility acceptance: automated keyboard/axe/reflow checks passed, real screen-reader/physical-device/actual zoom review remains. Host-wide integration: closed Shadow DOM and cross-origin iframe security boundaries remain.                                                                                                                                                                                                                                                                                                                                                                                                                          |
+| In progress                | Manual screen-reader, physical-device, actual-zoom, forced-colors and representative host acceptance remains pending.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
+| Planned                    | Richer localization and multi-language dictionary glossaries                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
+| Blocked                    | Local macOS Firefox launch only (Linux coverage passed); stable human acceptance remains pending                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
+| Deferred                   | Multi-provider arbitration and live cross-tab sync                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
+| Completed                  | Investigation/baseline, local P0 stabilization, feature regression suite, packed consumer/build verification, documentation, local example verification and full hosted CI meet their stated local acceptance criteria                                                                                                                                                                                                                                                                                                                                                                                                                                            |
 
 ## Stabilization and Clean light checks
 
diff --git a/docs/ROADMAP.md b/docs/ROADMAP.md
index 640ec21..b63fdde 100644
--- a/docs/ROADMAP.md
+++ b/docs/ROADMAP.md
@@ -2,37 +2,37 @@
 
 No deadlines or percentage-complete estimates. Status is based on current evidence in PROJECT_STATUS.md.
 
-| Phase                              | Priority | Objective / tasks                                                                                                 | Dependencies                           | Acceptance / deliverables                                                               | Risk / status                                                                            |
-| ---------------------------------- | -------- | ----------------------------------------------------------------------------------------------------------------- | -------------------------------------- | --------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------- |
-| Investigation                      | P0       | Compare vanilla/main/development/new-development, PR #1, history; baseline all checks                             | Git/registry access                    | Evidence-backed baseline and parity matrix                                              | Completed locally                                                                        |
-| Core stabilization                 | P0       | Provider effects, filter composition, independent storage, reset cancellation, no invisible hit area              | Baseline                               | Close/reopen, reset race, restore, dynamic content and cleanup regressions              | Implemented; local automated verification                                                |
-| Features and extension             | P1       | All fifteen controllers, missing original variants, safe custom apply contract                                    | Core                                   | Per-feature bounds/states and custom restore/reset/unmount tests; public examples       | Implemented; host-specific effect limits documented                                      |
-| Accessibility/UI                   | P1       | Non-modal semantics, names/focus, safe-area responsive CSS, motion preferences                                    | Core                                   | Keyboard/browser/axe checks plus human SR, actual zoom and device review                | Automated checks implemented; human review remains                                       |
-| Packaging/frameworks               | P1       | Client boundary, ESM/CJS/types/CSS, React 18/19, Next App/Pages, built examples                                   | Core + build                           | Clean tarball installs, production builds and runtime tests                             | Local and hosted consumer verification passed; complete Firefox coverage passed on Linux |
-| CI/release validation              | P1       | Execute hosted matrix; fix Firefox launch environment; record results                                             | Push/PR workflow and compatible runner | Passing Node22/24 jobs and full browser matrix on exact release commit                  | Completed: hosted Node22/24 and full Chromium/Firefox/WebKit/mobile matrix passed        |
-| Clean light base design            | P1       | Apply owner-selected light concept, icons, tokens and responsive rows                                             | Stabilized core                        | Browser/axe/target-size checks, actual runtime screenshots and documented overrides     | Merged through PR #16; local and hosted checks passed                                    |
-| Movement and quick actions         | P1       | Persist drag/corner placement, non-drag controls and fixed reset/footer                                           | Clean light foundation                 | Drag/cancel/reload/resize and pinned-action browser regressions                         | Merged through PR #17; local and full hosted checks passed                               |
-| Ownership/release                  | P1       | Continue confirmed npm identity; resolve license; choose version; approve merge/tag/publish                       | Owner decision + all gates             | Traceable authorized release with clean registry install                                | 2.0.0/latest published; automated release pipeline active for 2.1.0–2.22.0               |
-| Modular control primitives         | P1       | Accessible primitives (Toggle, Choice, Stepper, Card, Preview), search counts, empty/error UI states              | Clean light core                       | WCAG 2.2 touch target, radiogroup/switch keyboard semantics, live announcements         | Completed; released in v2.3.0                                                            |
-| Typography enhancements            | P1       | TextScale (stepwise), LineHeight presets, script-aware LetterSpacing and WordSpacing, ParagraphSpacing            | Core text engine                       | Arabic cursive protection, Sinhala/Tamil script bounds, reflow safety                   | Completed; released in v2.4.0–v2.9.0                                                     |
-| Reading aids & Media hiding        | P1       | ReadingGuide, ReadingMask, BigCursor fine-pointer detection, HideDecorativeImages, HideVideos                     | Portal layout                          | Safe-area fit, pointer-transparency, alt text preservation, performance                 | Completed; released in v2.10.0–v2.14.0                                                   |
-| Navigation, Presets & I18n         | P1       | PageNavigator, Tooltips (WCAG 1.4.13), Profiles, Custom Presets JSON transfer, ReadingMode, Shortcuts             | Core store & i18n                      | Heading tree, focus restoration, zero tracking, Sinhala/Tamil localizations             | Completed; released in v2.15.0–v2.19.0                                                   |
-| Native Speech TTS (Issue 57)       | P1       | User-initiated Web Speech read-aloud with sentence chunking, pitch/rate, voice selector, floating bar             | Web Speech API                         | Zero network calls, sensitive field exclusions, no auto-play                            | Completed; released in v2.20.0                                                           |
-| Saturation/Grayscale (58)          | P1       | Opt-in saturation levels, monochrome grayscale, image/chart advisory warnings, forced-colors bypass               | Filter pipeline                        | Non-destructive reset, high-contrast OS detection, accessible advisories                | Completed; released in v2.21.0                                                           |
-| Menu Customization (Issue 60)      | P1       | Per-site feature menu reordering, grouping, custom labels, safe diagnostics, and stable rerenders                 | AccessibilityList / FeatureMenu        | Non-crashing error handling, preserved canonical IDs, memoized focus stability          | Completed; released in v2.23.0                                                           |
-| Diagnostics & Harness (61)         | P1       | Dev-only integration diagnostics, containing-block traps, CSP, host CSS, DOM landmarks and test harness           | Diagnostics engine & harness           | Zero compliance claims, strict privacy guarantees, actionable fixes                     | Completed; released in v2.24.0                                                           |
-| Smart Contrast Prototype (63)      | P1       | Measured smart contrast for eligible elements with compositing, caching, unsupported reporting, full restore      | Smart contrast engine & controller     | Non-destructive restoration, no layout thrashing, Porter-Duff compositing, zero claims  | Completed; released in v2.26.0                                                           |
-| Voice Navigation Prototype (64)    | P1       | Client-side voice navigation for headings and scrolling with timeout, tab cleanup, zero recording storage         | Web Speech Recognition & controller    | Inactivity timeout, document.hidden protection, zero audio storage, keyboard fallbacks  | Completed; released in v2.27.0                                                           |
-| Host Translation Adapter (65)      | P1       | Decoupled translation provider interface, consent modal, sensitive field exclusion, DOM restoration               | Translation engine & controller        | Explicit consent, privacy threat model, zero default requests, textContent sanitization | Completed; released in v2.28.0                                                           |
-| Compatibility Matrix (Issue 66)    | P1       | Full supported-browser, device, zoom reflow, forced-colors, safe-area and touch-target matrix                     | Test suite & documentation             | 320px 400% reflow, forced-colors bypass, 44px targets, zero overclaims                  | Completed; documented in docs/SUPPORTED_MATRIX.md                                        |
-| Performance Profiling (Issue 67)   | P1       | Measure bundle gzip, closed runtime, 1,000-node DOM jank, 100 mount/unmount and toggle cycles                     | Performance test suite & doc           | Actual gzip/brotli metrics, <45ms main thread, 0 leaks, 0 compounding                   | Completed; documented in docs/PERFORMANCE_PROFILING.md                                   |
-| Documentation & Demos (Issue 68)   | P1       | Synchronize README, API, FEATURES, CONTRIBUTING, and runnable Vite/Next consumer examples                         | Documentation and examples             | Complete API & features sync, CONTRIBUTING guide, full runnable package demos           | Completed; documented in CONTRIBUTING.md, docs/API.md, docs/FEATURES.md                  |
-| Security & Audit (Issue 69)        | P1       | Audit dependencies, licenses, NOTICE, CSP, unsafe eval/HTML, privacy model, and vulnerabilities                   | Security & audit test suite            | 0 dependencies, 0 audit advisories, strict CSP compliance, sensitive data protection    | Completed; documented in NOTICE, docs/SECURITY_AUDIT.md                                  |
-| Publish Rehearsal (Issue 70)       | P1       | Pack dry-run archive inspection, clean React 18/19 and Next.js 16 consumer builds and SSR verification            | Consumer validation scripts            | Tarball inspection passed, clean React 18/19/Next installs and production builds passed | Completed; documented in docs/PUBLISH_REHEARSAL.md                                       |
-| Release Proposal (Issue 71)        | P1       | Review evidence, recommend patch/minor, prepare dated changelog, reconcile #15, and await owner approval          | Release proposal & versioning analysis | Comprehensive evidence review, SemVer recommendation, legacy tracker reconciliation     | Completed; documented in docs/RELEASE_PROPOSAL_M5.md                                     |
-| Large Document & Font Refresh (12) | P1       | Profile 8,000-node DOMs, incremental child mutations (0 FOUC), document.fonts refresh, root attribute observation | Core effects engine                    | Measured benchmarks, incremental batching, font event listener, no observer loops       | Completed; documented in docs/LARGE_DOCUMENT_PROFILING.md                                |
-| Host effects hardening             | P2       | Optional explicit effect scopes, CSP renderer, Shadow DOM strategy, fixed-descendant filter alternatives          | API design and real use cases          | Documented contract with package/browser tests                                          | Planned; avoid universal-host compatibility claims                                       |
-| Multi-instance/cross-tab           | P2       | Decide if needed; deterministic arbitration/synchronization                                                       | Usage evidence                         | Isolated lifetime, persistence and interaction tests                                    | Deferred; one provider/document enforced                                                 |
+| Phase                              | Priority | Objective / tasks                                                                                                     | Dependencies                           | Acceptance / deliverables                                                               | Risk / status                                                                            |
+| ---------------------------------- | -------- | --------------------------------------------------------------------------------------------------------------------- | -------------------------------------- | --------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------- |
+| Investigation                      | P0       | Compare vanilla/main/development/new-development, PR #1, history; baseline all checks                                 | Git/registry access                    | Evidence-backed baseline and parity matrix                                              | Completed locally                                                                        |
+| Core stabilization                 | P0       | Provider effects, filter composition, independent storage, reset cancellation, no invisible hit area                  | Baseline                               | Close/reopen, reset race, restore, dynamic content and cleanup regressions              | Implemented; local automated verification                                                |
+| Features and extension             | P1       | All fifteen controllers, missing original variants, safe custom apply contract                                        | Core                                   | Per-feature bounds/states and custom restore/reset/unmount tests; public examples       | Implemented; host-specific effect limits documented                                      |
+| Accessibility/UI                   | P1       | Non-modal semantics, names/focus, safe-area responsive CSS, motion preferences                                        | Core                                   | Keyboard/browser/axe checks plus human SR, actual zoom and device review                | Automated checks implemented; human review remains                                       |
+| Packaging/frameworks               | P1       | Client boundary, ESM/CJS/types/CSS, React 18/19, Next App/Pages, built examples                                       | Core + build                           | Clean tarball installs, production builds and runtime tests                             | Local and hosted consumer verification passed; complete Firefox coverage passed on Linux |
+| CI/release validation              | P1       | Execute hosted matrix; fix Firefox launch environment; record results                                                 | Push/PR workflow and compatible runner | Passing Node22/24 jobs and full browser matrix on exact release commit                  | Completed: hosted Node22/24 and full Chromium/Firefox/WebKit/mobile matrix passed        |
+| Clean light base design            | P1       | Apply owner-selected light concept, icons, tokens and responsive rows                                                 | Stabilized core                        | Browser/axe/target-size checks, actual runtime screenshots and documented overrides     | Merged through PR #16; local and hosted checks passed                                    |
+| Movement and quick actions         | P1       | Persist drag/corner placement, non-drag controls and fixed reset/footer                                               | Clean light foundation                 | Drag/cancel/reload/resize and pinned-action browser regressions                         | Merged through PR #17; local and full hosted checks passed                               |
+| Ownership/release                  | P1       | Continue confirmed npm identity; resolve license; choose version; approve merge/tag/publish                           | Owner decision + all gates             | Traceable authorized release with clean registry install                                | 2.0.0/latest published; automated release pipeline active for 2.1.0–2.22.0               |
+| Modular control primitives         | P1       | Accessible primitives (Toggle, Choice, Stepper, Card, Preview), search counts, empty/error UI states                  | Clean light core                       | WCAG 2.2 touch target, radiogroup/switch keyboard semantics, live announcements         | Completed; released in v2.3.0                                                            |
+| Typography enhancements            | P1       | TextScale (stepwise), LineHeight presets, script-aware LetterSpacing and WordSpacing, ParagraphSpacing                | Core text engine                       | Arabic cursive protection, Sinhala/Tamil script bounds, reflow safety                   | Completed; released in v2.4.0–v2.9.0                                                     |
+| Reading aids & Media hiding        | P1       | ReadingGuide, ReadingMask, BigCursor fine-pointer detection, HideDecorativeImages, HideVideos                         | Portal layout                          | Safe-area fit, pointer-transparency, alt text preservation, performance                 | Completed; released in v2.10.0–v2.14.0                                                   |
+| Navigation, Presets & I18n         | P1       | PageNavigator, Tooltips (WCAG 1.4.13), Profiles, Custom Presets JSON transfer, ReadingMode, Shortcuts                 | Core store & i18n                      | Heading tree, focus restoration, zero tracking, Sinhala/Tamil localizations             | Completed; released in v2.15.0–v2.19.0                                                   |
+| Native Speech TTS (Issue 57)       | P1       | User-initiated Web Speech read-aloud with sentence chunking, pitch/rate, voice selector, floating bar                 | Web Speech API                         | Zero network calls, sensitive field exclusions, no auto-play                            | Completed; released in v2.20.0                                                           |
+| Saturation/Grayscale (58)          | P1       | Opt-in saturation levels, monochrome grayscale, image/chart advisory warnings, forced-colors bypass                   | Filter pipeline                        | Non-destructive reset, high-contrast OS detection, accessible advisories                | Completed; released in v2.21.0                                                           |
+| Menu Customization (Issue 60)      | P1       | Per-site feature menu reordering, grouping, custom labels, safe diagnostics, and stable rerenders                     | AccessibilityList / FeatureMenu        | Non-crashing error handling, preserved canonical IDs, memoized focus stability          | Completed; released in v2.23.0                                                           |
+| Diagnostics & Harness (61)         | P1       | Dev-only integration diagnostics, containing-block traps, CSP, host CSS, DOM landmarks and test harness               | Diagnostics engine & harness           | Zero compliance claims, strict privacy guarantees, actionable fixes                     | Completed; released in v2.24.0                                                           |
+| Smart Contrast Prototype (63)      | P1       | Measured smart contrast for eligible elements with compositing, caching, unsupported reporting, full restore          | Smart contrast engine & controller     | Non-destructive restoration, no layout thrashing, Porter-Duff compositing, zero claims  | Completed; released in v2.26.0                                                           |
+| Voice Navigation Prototype (64)    | P1       | Client-side voice navigation for headings and scrolling with timeout, tab cleanup, zero recording storage             | Web Speech Recognition & controller    | Inactivity timeout, document.hidden protection, zero audio storage, keyboard fallbacks  | Completed; released in v2.27.0                                                           |
+| Host Translation Adapter (65)      | P1       | Decoupled translation provider interface, consent modal, sensitive field exclusion, DOM restoration                   | Translation engine & controller        | Explicit consent, privacy threat model, zero default requests, textContent sanitization | Completed; released in v2.28.0                                                           |
+| Compatibility Matrix (Issue 66)    | P1       | Full supported-browser, device, zoom reflow, forced-colors, safe-area and touch-target matrix                         | Test suite & documentation             | 320px 400% reflow, forced-colors bypass, 44px targets, zero overclaims                  | Completed; documented in docs/SUPPORTED_MATRIX.md                                        |
+| Performance Profiling (Issue 67)   | P1       | Measure bundle gzip, closed runtime, 1,000-node DOM jank, 100 mount/unmount and toggle cycles                         | Performance test suite & doc           | Actual gzip/brotli metrics, <45ms main thread, 0 leaks, 0 compounding                   | Completed; documented in docs/PERFORMANCE_PROFILING.md                                   |
+| Documentation & Demos (Issue 68)   | P1       | Synchronize README, API, FEATURES, CONTRIBUTING, and runnable Vite/Next consumer examples                             | Documentation and examples             | Complete API & features sync, CONTRIBUTING guide, full runnable package demos           | Completed; documented in CONTRIBUTING.md, docs/API.md, docs/FEATURES.md                  |
+| Security & Audit (Issue 69)        | P1       | Audit dependencies, licenses, NOTICE, CSP, unsafe eval/HTML, privacy model, and vulnerabilities                       | Security & audit test suite            | 0 dependencies, 0 audit advisories, strict CSP compliance, sensitive data protection    | Completed; documented in NOTICE, docs/SECURITY_AUDIT.md                                  |
+| Publish Rehearsal (Issue 70)       | P1       | Pack dry-run archive inspection, clean React 18/19 and Next.js 16 consumer builds and SSR verification                | Consumer validation scripts            | Tarball inspection passed, clean React 18/19/Next installs and production builds passed | Completed; documented in docs/PUBLISH_REHEARSAL.md                                       |
+| Release Proposal (Issue 71)        | P1       | Review evidence, recommend patch/minor, prepare dated changelog, reconcile #15, and await owner approval              | Release proposal & versioning analysis | Comprehensive evidence review, SemVer recommendation, legacy tracker reconciliation     | Completed; documented in docs/RELEASE_PROPOSAL_M5.md                                     |
+| Large Document & Font Refresh (12) | P1       | Profile 8,000-node DOMs, incremental child mutations (0 FOUC), document.fonts refresh, root attribute observation     | Core effects engine                    | Measured benchmarks, incremental batching, font event listener, no observer loops       | Completed; documented in docs/LARGE_DOCUMENT_PROFILING.md                                |
+| Host effects hardening (Issue 13)  | P2       | Explicit effect scopes (effectScope), strict CSP nonce, open Shadow DOM traversal, fixed-descendant filter protection | Core effects engine & API              | Documented contract with package/browser tests                                          | Completed; documented in docs/SCOPED_EFFECTS.md                                          |
+| Multi-instance/cross-tab           | P2       | Decide if needed; deterministic arbitration/synchronization                                                           | Usage evidence                         | Isolated lifetime, persistence and interaction tests                                    | Deferred; one provider/document enforced                                                 |
 
 Release gates are not satisfied by checking in tests or CI configuration. Execute them on the final candidate. A local Chromium/WebKit pass is not a Firefox pass, and an axe pass is not screen-reader or WCAG certification.
 
diff --git a/docs/SCOPED_EFFECTS.md b/docs/SCOPED_EFFECTS.md
new file mode 100644
index 0000000..80229be
--- /dev/null
+++ b/docs/SCOPED_EFFECTS.md
@@ -0,0 +1,129 @@
+# Scoped Effects, Strict CSP, and Host Compatibility (Issue #13)
+
+This document specifies the design, contracts, and platform boundaries for scoped effect targeting (`effectScope`), strict Content Security Policy nonce support (`nonce`), open Shadow DOM traversal, and host container isolation in `accessibility-plugin-react`.
+
+---
+
+## 1. Overview & Motivation
+
+By default, `accessibility-plugin-react` operates host-wide, traversing eligible descendants of `document.body` and applying display enhancements (font enlargement, proportional line height, letter and word spacing, semantic contrast, and color filters).
+
+In complex or enterprise host applications, host-wide styling presents two primary challenges:
+
+1. **Host Chrome & Navigation Bleed**: Embedded sub-portals, micro-frontends, multi-tenant dashboards, and CMS content viewers often need accessibility preferences restricted to their main content region (e.g., `<main id="page-content">`), leaving application top bars, breadcrumbs, user avatars, or side navigation untouched.
+2. **Fixed-Descendant Filter Stacking Contexts**: According to the CSS Transforms and Filter Effects Level 1 specifications, applying a CSS filter (`filter: grayscale(...)` or `filter: invert(...)`) to any element causes that element to become a containing block and stacking context for all `position: fixed` descendants. In host layouts with fixed headers or sticky navbars, root filtering can cause fixed elements to jump or lose viewport pinning.
+
+---
+
+## 2. Scoped Effects Configuration (`effectScope`)
+
+### Configuration Prop
+
+`AccessibilityPlugin` supports the optional `effectScope` prop:
+
+```tsx
+import {
+  AccessibilityPlugin,
+  AccessibilityButton,
+  AccessibilityList,
+  TextScaleController,
+} from "accessibility-plugin-react";
+
+export function App() {
+  const contentRef = useRef<HTMLElement>(null);
+
+  return (
+    <AccessibilityPlugin effectScope="#main-content">
+      {/* Host navigation remains completely untouched by accessibility preferences */}
+      <header className="fixed top-0 left-0 w-full z-50">
+        <Navbar />
+      </header>
+
+      {/* Preferences apply strictly to this container and its descendants */}
+      <main id="main-content" ref={contentRef}>
+        <Article />
+      </main>
+
+      <AccessibilityButton />
+      <AccessibilityList>
+        <TextScaleController />
+      </AccessibilityList>
+    </AccessibilityPlugin>
+  );
+}
+```
+
+### Supported Target Types
+
+`effectScope` accepts any of the following types (`EffectScopeTarget`):
+
+| Type                               | Example                              | Resolution Behavior                                                                                              |
+| :--------------------------------- | :----------------------------------- | :--------------------------------------------------------------------------------------------------------------- |
+| **CSS Selector String**            | `"#main-content"`, `".article-body"` | Resolves via `document.querySelector<HTMLElement>(selector)`. Falls back safely to `document.body` if not found. |
+| **HTMLElement**                    | `document.getElementById("main")`    | Uses the passed element directly.                                                                                |
+| **React RefObject**                | `useRef<HTMLElement>(null)`          | Resolves `ref.current`. Falls back to `document.body` if unmounted or null.                                      |
+| **Omitted / `undefined` / `null`** | `undefined`                          | Defaults to `document.body` (100% backward compatible).                                                          |
+
+### Scoped Container Filter Isolation
+
+When `effectScope` targets a container (e.g. `<main id="main-content">`):
+
+- CSS filters (grayscale, invert, saturation) are applied **directly to the scoped container**, rather than filtering every child of `document.body`.
+- External fixed elements outside the scoped container (such as `<header className="fixed top-0">` or sticky sidebars) receive **zero CSS filters**, completely eliminating filter-induced containing-block positioning bugs.
+- The widget portal itself (`.arap-root`) remains excluded from filtering and is portaled outside the container.
+
+---
+
+## 3. Open Shadow DOM Traversal
+
+Modern web applications may incorporate Web Components or custom elements using Shadow DOM.
+
+- **Open Shadow DOM (`mode: "open"`)**:
+  - Traversal scans elements inside `scopeRoot`. When an element has `.shadowRoot` with `mode === "open"`, the query engine traverses into `shadowRoot.querySelectorAll("*")` and applies typography and display preferences to eligible shadow descendants.
+  - Incremental mutation batching also traverses open shadow roots when new nodes are added.
+- **Closed Shadow DOM (`mode: "closed"`)**:
+  - Closed shadow roots are completely inaccessible to external scripts by W3C platform security design (`element.shadowRoot` evaluates to `null`). Closed shadow DOM contents remain isolated.
+
+---
+
+## 4. iframe Platform Boundaries
+
+- **Cross-Origin iframes**:
+  - Inaccessible due to browser Same-Origin Policy (SOP). Attempts to access `iframe.contentDocument` or modify internal styles throw security errors.
+  - The plugin strictly respects SOP boundaries and never attempts cross-origin DOM traversal.
+- **Same-Origin iframes**:
+  - Same-origin iframes possess their own distinct browsing contexts and style cascades. Styling an iframe's internal DOM requires mounting an `AccessibilityPlugin` within that iframe's window context.
+
+---
+
+## 5. Strict Content Security Policy (CSP) & Nonces
+
+### Nonce Support (`nonce`)
+
+In environments enforcing strict CSP with `style-src 'self' 'nonce-...'` (disallowing `'unsafe-inline'`):
+
+```tsx
+<AccessibilityPlugin nonce={nonceValue}>
+  <App />
+</AccessibilityPlugin>
+```
+
+When provided:
+
+1. All dynamically injected `<style>` elements (such as `arap-focus-highlight-style` and `arap-reduce-motion-style`) have their `nonce` attribute and property assigned (`styleEl.nonce = nonce; styleEl.setAttribute("nonce", nonce)`).
+2. The browser permits stylesheet execution under strict nonce-based policies.
+3. If `nonce` is omitted and CSP forbids inline `<style>`, consumers can import the precompiled stylesheet statically (`import "accessibility-plugin-react/style.css"`).
+
+### Custom SVG Cursor & Fallback
+
+The `bigCursor` feature uses an inline SVG data URI (`data:image/svg+xml,...`).
+
+- Requires `img-src 'self' data:`.
+- If `data:` is omitted from `img-src`, modern browsers block the custom cursor and automatically fall back to the native system pointer with zero JavaScript exceptions.
+
+---
+
+## 6. Backward Compatibility & Migration
+
+- **Zero Breaking Changes**: When `effectScope` and `nonce` are omitted, behavior is 100% identical to prior releases.
+- **Full Restoration**: `resetAll()`, `session.undo()`, or component unmount cleanly restores all previous inline styles, filters, and priorities on both the scoped container and all modified descendants.
diff --git a/src/core/AccessibilityPlugin.tsx b/src/core/AccessibilityPlugin.tsx
index fca7359..4b95db2 100644
--- a/src/core/AccessibilityPlugin.tsx
+++ b/src/core/AccessibilityPlugin.tsx
@@ -32,6 +32,8 @@ export function AccessibilityPlugin({
   largeControls,
   diagnostics,
   onFeatureEvent,
+  effectScope,
+  nonce,
 }: AccessibilityPluginProps) {
   const [isOpen, setIsOpen] = useState(false);
   const [store] = useState(() =>
@@ -312,7 +314,7 @@ export function AccessibilityPlugin({
       : undefined) ||
     undefined;
 
-  useBuiltinEffects(active, handleDisable);
+  useBuiltinEffects(active, handleDisable, { effectScope, nonce });
   const value = useMemo(
     () => ({
       state: { isOpen, features },
@@ -332,6 +334,8 @@ export function AccessibilityPlugin({
       locale: effectiveLocale,
       statementUrl,
       largeControls,
+      effectScope,
+      nonce,
     }),
     [
       isOpen,
@@ -351,6 +355,8 @@ export function AccessibilityPlugin({
       effectiveLocale,
       statementUrl,
       largeControls,
+      effectScope,
+      nonce,
     ],
   );
   return (
diff --git a/src/core/effects.ts b/src/core/effects.ts
index a032d9b..0ef82c8 100644
--- a/src/core/effects.ts
+++ b/src/core/effects.ts
@@ -730,10 +730,48 @@ export function isEligibleDecorativeImage(el: Element): boolean {
   return false;
 }
 
+export type EffectScopeTarget =
+  string | HTMLElement | { current: HTMLElement | null } | null | undefined;
+
+export interface ApplyPageOptions {
+  effectScope?: EffectScopeTarget;
+  nonce?: string;
+}
+
 export interface EffectsSession {
   undo: () => void;
   applyIncremental: (addedNodes: HTMLElement[]) => void;
   styledCount: () => number;
+  targetRoot?: HTMLElement | null;
+}
+
+export function resolveEffectScope(
+  scope: EffectScopeTarget,
+): HTMLElement | null {
+  if (typeof document === "undefined") return null;
+  if (!scope) return document.body ?? null;
+  if (typeof scope === "string") {
+    try {
+      return (
+        document.querySelector<HTMLElement>(scope) ?? document.body ?? null
+      );
+    } catch {
+      return document.body ?? null;
+    }
+  }
+  if (typeof scope === "object") {
+    if ("nodeType" in scope && (scope as Node).nodeType === 1) {
+      return scope as HTMLElement;
+    }
+    if ("current" in scope) {
+      return (
+        (scope as { current: HTMLElement | null }).current ??
+        document.body ??
+        null
+      );
+    }
+  }
+  return document.body ?? null;
 }
 
 function applyToElements(
@@ -1107,7 +1145,29 @@ function applyToElements(
 
 export function applyPageSession(
   values: Record<string, unknown>,
+  optionsOrScope?: EffectScopeTarget | ApplyPageOptions,
 ): EffectsSession {
+  let effectScope: EffectScopeTarget = undefined;
+  let nonce: string | undefined = undefined;
+
+  if (optionsOrScope && typeof optionsOrScope === "object") {
+    if ("nodeType" in optionsOrScope) {
+      effectScope = optionsOrScope as HTMLElement;
+    } else if (
+      "current" in optionsOrScope &&
+      !("effectScope" in optionsOrScope)
+    ) {
+      effectScope = optionsOrScope as { current: HTMLElement | null };
+    } else {
+      const opts = optionsOrScope as ApplyPageOptions;
+      effectScope = opts.effectScope;
+      nonce = opts.nonce;
+    }
+  } else if (typeof optionsOrScope === "string") {
+    effectScope = optionsOrScope;
+  }
+
+  const targetRoot = resolveEffectScope(effectScope);
   const undo: (() => void)[] = [];
   const styledElements = new WeakSet<HTMLElement>();
   let totalStyled = 0;
@@ -1138,7 +1198,10 @@ export function applyPageSession(
       computed.backgroundColor === "rgba(0, 0, 0, 0)" ||
       computed.backgroundColor === "transparent"
     ) {
-      const bodyColor = getComputedStyle(document.body).backgroundColor;
+      const bodyColor =
+        typeof document !== "undefined" && document.body
+          ? getComputedStyle(document.body).backgroundColor
+          : "#ffffff";
       set(
         el,
         "background-color",
@@ -1153,10 +1216,18 @@ export function applyPageSession(
     set(el, "filter", `${base && base !== "none" ? base + " " : ""}${filter}`);
   }
 
-  // Never filter html/body: they would also transform fixed widget positioning.
-  if (filter && typeof document !== "undefined" && document.body) {
-    for (const el of Array.from(document.body.children)) {
-      if (el instanceof HTMLElement) applyContainerFilter(el);
+  // Never filter html/body directly: they would also transform fixed widget positioning.
+  if (filter && targetRoot) {
+    if (
+      targetRoot === (typeof document !== "undefined" ? document.body : null)
+    ) {
+      for (const el of Array.from(document.body.children)) {
+        if (el instanceof HTMLElement) applyContainerFilter(el);
+      }
+    } else {
+      // Scoped container: apply filter directly to targetRoot!
+      // This protects host fixed elements (navbars, modals) outside targetRoot from filter stacking contexts.
+      applyContainerFilter(targetRoot);
     }
   }
 
@@ -1183,6 +1254,10 @@ export function applyPageSession(
       styleEl = document.createElement("style");
       styleEl.id = FOCUS_STYLE_ID;
       styleEl.dataset.arapUi = "";
+      if (nonce) {
+        styleEl.nonce = nonce;
+        styleEl.setAttribute("nonce", nonce);
+      }
       styleEl.textContent = `
         *:not([data-arap-ui]):not([data-arap-ui] *):focus-visible {
           outline: 3px solid #2563eb !important;
@@ -1219,6 +1294,10 @@ export function applyPageSession(
       styleEl = document.createElement("style");
       styleEl.id = MOTION_STYLE_ID;
       styleEl.dataset.arapUi = "";
+      if (nonce) {
+        styleEl.nonce = nonce;
+        styleEl.setAttribute("nonce", nonce);
+      }
       styleEl.textContent = `
         *:not([data-arap-ui]):not([data-arap-ui] *),
         *:not([data-arap-ui]):not([data-arap-ui] *)::before,
@@ -1293,16 +1372,33 @@ export function applyPageSession(
     values.bigCursor &&
     isFinePointer &&
     typeof document !== "undefined" &&
-    document.body
+    targetRoot
   ) {
-    set(document.body, "cursor", cursor);
+    set(targetRoot, "cursor", cursor);
   }
 
   // Initial elements application up to MAX_HOST_ELEMENTS
-  if (typeof document !== "undefined" && document.body) {
-    const allElements = Array.from(
-      document.body.querySelectorAll<HTMLElement>("*"),
+  if (typeof document !== "undefined" && targetRoot) {
+    const allElements: HTMLElement[] = [];
+    if (targetRoot !== document.body && !targetRoot.matches(EXCLUDED)) {
+      allElements.push(targetRoot);
+    }
+    const descendants = Array.from(
+      targetRoot.querySelectorAll<HTMLElement>("*"),
     ).filter((el) => el instanceof HTMLElement && !el.matches(EXCLUDED));
+    allElements.push(...descendants);
+
+    // Open Shadow DOM traversal
+    for (const el of descendants) {
+      if (allElements.length >= MAX_HOST_ELEMENTS) break;
+      if (el.shadowRoot && el.shadowRoot.mode === "open") {
+        const shadowDescendants = Array.from(
+          el.shadowRoot.querySelectorAll<HTMLElement>("*"),
+        ).filter((s) => s instanceof HTMLElement && !s.matches(EXCLUDED));
+        allElements.push(...shadowDescendants);
+      }
+    }
+
     const elements =
       allElements.length > MAX_HOST_ELEMENTS
         ? allElements.slice(0, MAX_HOST_ELEMENTS)
@@ -1313,14 +1409,28 @@ export function applyPageSession(
   }
 
   function applyIncremental(addedNodes: HTMLElement[]) {
-    if (typeof document === "undefined" || !document.body) return;
+    if (typeof document === "undefined" || !targetRoot) return;
 
     const candidateElements: HTMLElement[] = [];
     for (const root of addedNodes) {
       if (root.matches(EXCLUDED)) continue;
 
-      // If a new direct child was appended to body and container filter is active, apply it
-      if (filter && root.parentElement === document.body) {
+      // If scoped, ignore any mutations outside targetRoot
+      if (
+        targetRoot !== document.body &&
+        !targetRoot.contains(root) &&
+        root !== targetRoot
+      ) {
+        continue;
+      }
+
+      // If a new direct child was appended to body (or targetRoot) and container filter is active, apply it
+      if (
+        filter &&
+        (targetRoot === document.body
+          ? root.parentElement === document.body
+          : root === targetRoot || root.parentElement === targetRoot)
+      ) {
         applyContainerFilter(root);
       }
 
@@ -1330,6 +1440,15 @@ export function applyPageSession(
         root.querySelectorAll<HTMLElement>("*"),
       ).filter((el) => el instanceof HTMLElement && !el.matches(EXCLUDED));
       candidateElements.push(...descendants);
+
+      for (const el of descendants) {
+        if (el.shadowRoot && el.shadowRoot.mode === "open") {
+          const shadowDescendants = Array.from(
+            el.shadowRoot.querySelectorAll<HTMLElement>("*"),
+          ).filter((s) => s instanceof HTMLElement && !s.matches(EXCLUDED));
+          candidateElements.push(...shadowDescendants);
+        }
+      }
     }
 
     if (candidateElements.length > 0) {
@@ -1358,11 +1477,15 @@ export function applyPageSession(
     },
     applyIncremental,
     styledCount: () => totalStyled,
+    targetRoot,
   };
 }
 
-export function applyPage(values: Record<string, unknown>): () => void {
-  const session = applyPageSession(values);
+export function applyPage(
+  values: Record<string, unknown>,
+  optionsOrScope?: EffectScopeTarget | ApplyPageOptions,
+): () => void {
+  const session = applyPageSession(values, optionsOrScope);
   return session.undo;
 }
 
@@ -1552,6 +1675,7 @@ function readingAid(
 export function useBuiltinEffects(
   values: Record<string, unknown>,
   onDisable?: (featureKey: string) => void,
+  optionsOrScope?: EffectScopeTarget | ApplyPageOptions,
 ) {
   useEffect(() => {
     if (
@@ -1562,7 +1686,15 @@ export function useBuiltinEffects(
     )
       return;
 
-    let activeSession = applyPageSession(values);
+    const targetRoot = resolveEffectScope(
+      optionsOrScope &&
+        typeof optionsOrScope === "object" &&
+        "effectScope" in optionsOrScope
+        ? (optionsOrScope as ApplyPageOptions).effectScope
+        : (optionsOrScope as EffectScopeTarget),
+    );
+
+    let activeSession = applyPageSession(values, optionsOrScope);
     let refreshTimer: ReturnType<typeof setTimeout> | null = null;
     let incrementalTimer: ReturnType<typeof setTimeout> | null = null;
     let pendingAddedNodes: HTMLElement[] = [];
@@ -1578,7 +1710,7 @@ export function useBuiltinEffects(
       if (refreshTimer) clearTimeout(refreshTimer);
       refreshTimer = setTimeout(() => {
         activeSession.undo();
-        activeSession = applyPageSession(values);
+        activeSession = applyPageSession(values, optionsOrScope);
       }, delay);
     };
 
@@ -1591,7 +1723,8 @@ export function useBuiltinEffects(
         if (record.type === "attributes") {
           if (
             record.target === document.documentElement ||
-            record.target === document.body
+            record.target === document.body ||
+            record.target === targetRoot
           ) {
             hasRootAttributeChange = true;
           }
@@ -1625,13 +1758,27 @@ export function useBuiltinEffects(
       }
     });
 
-    const bodyObserverOptions: MutationObserverInit = {
+    const observerOptions: MutationObserverInit = {
       childList: true,
       subtree: true,
       attributes: true,
       attributeFilter: ["class", "data-theme", "dir", "lang"],
     };
-    observer.observe(document.body, bodyObserverOptions);
+
+    if (targetRoot) {
+      observer.observe(targetRoot, observerOptions);
+    }
+
+    if (
+      typeof document !== "undefined" &&
+      document.body &&
+      targetRoot !== document.body
+    ) {
+      observer.observe(document.body, {
+        attributes: true,
+        attributeFilter: ["class", "data-theme", "dir", "lang"],
+      });
+    }
 
     if (typeof document !== "undefined" && document.documentElement) {
       observer.observe(document.documentElement, {
@@ -1681,7 +1828,7 @@ export function useBuiltinEffects(
       window.removeEventListener("resize", resize);
       activeSession.undo();
     };
-  }, [values]);
+  }, [values, optionsOrScope]);
 
   useEffect(() => {
     if (!values.readingMask) return;
diff --git a/src/core/types.ts b/src/core/types.ts
index 403ff41..e6aec35 100644
--- a/src/core/types.ts
+++ b/src/core/types.ts
@@ -32,6 +32,8 @@ export interface AccessibilityContextValue {
   locale?: string;
   statementUrl?: string;
   largeControls?: boolean;
+  effectScope?: string | HTMLElement | { current: HTMLElement | null } | null;
+  nonce?: string;
 }
 export type WidgetPlacement =
   | {
@@ -83,6 +85,17 @@ export interface AccessibilityPluginProps {
    * inputs, transcripts, and identifiers.
    */
   onFeatureEvent?: FeatureEventListener;
+  /**
+   * Optional scoped container target for built-in effects (Issue #13).
+   * Can be a CSS selector string (e.g. '#main-content'), an HTMLElement,
+   * or a React ref object. Defaults to document.body when omitted.
+   */
+  effectScope?: string | HTMLElement | { current: HTMLElement | null } | null;
+  /**
+   * Optional Content Security Policy (CSP) nonce attribute passed to any
+   * dynamically injected <style> tags (Issue #13).
+   */
+  nonce?: string;
 }
 export interface AccessibilityButtonProps {
   /** Pointer dragging and Alt+Arrow movement. Defaults to true. */
diff --git a/src/index.ts b/src/index.ts
index d3c4006..3138d6d 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -110,7 +110,10 @@ export {
   MAX_HOST_ELEMENTS,
   applyPage,
   applyPageSession,
+  resolveEffectScope,
   type EffectsSession,
+  type EffectScopeTarget,
+  type ApplyPageOptions,
 } from "./core/effects";
 export {
   FONT_FAMILY_PRESETS,
diff --git a/tests/browser/Fixture.jsx b/tests/browser/Fixture.jsx
index 3c98fcb..b885acd 100644
--- a/tests/browser/Fixture.jsx
+++ b/tests/browser/Fixture.jsx
@@ -1,6 +1,9 @@
 "use client";
 import React, { useState } from "react";
 import * as A from "accessibility-plugin-react";
+if (typeof window !== "undefined") {
+  window.__ARAP__ = A;
+}
 function Custom() {
   const { value, setValue } = A.useAccessibility("hostOutline", false, {
     apply: (value) => {
diff --git a/tests/browser/widget.spec.ts b/tests/browser/widget.spec.ts
index 1f56d5e..01ab72b 100644
--- a/tests/browser/widget.spec.ts
+++ b/tests/browser/widget.spec.ts
@@ -1115,6 +1115,83 @@ test("Issue #12: large DOM incremental mutations, late font load, and root attri
   });
 });
 
+test("Issue #13: scoped effects, strict CSP, and host compatibility in real browser", async ({
+  page,
+}) => {
+  await page.goto("http://127.0.0.1:7311/");
+
+  // Set up scoped host layout dynamically
+  await page.evaluate(() => {
+    const existing = document.getElementById("issue-13-browser-host");
+    if (existing) existing.remove();
+
+    const host = document.createElement("div");
+    host.id = "issue-13-browser-host";
+    host.innerHTML = `
+      <header id="issue-13-external-header" style="position: fixed; top: 0; left: 0; font-size: 16px;">
+        <h2 class="ext-h2" style="font-size: 16px;">External Header</h2>
+      </header>
+      <main id="issue-13-scoped-main" style="margin-top: 60px; font-size: 16px;">
+        <p class="scoped-p" style="font-size: 16px;">Scoped main paragraph</p>
+      </main>
+    `;
+    document.body.appendChild(host);
+  });
+
+  // Apply scoped effects via window.__ARAP__.applyPageSession
+  await page.evaluate(() => {
+    const pkg = (
+      window as unknown as {
+        __ARAP__: {
+          applyPageSession: (v: unknown, o: unknown) => { undo: () => void };
+        };
+      }
+    ).__ARAP__;
+    if (pkg && pkg.applyPageSession) {
+      (
+        window as unknown as { __issue13_session: { undo: () => void } }
+      ).__issue13_session = pkg.applyPageSession(
+        { textScale: 1.25, grayscale: true },
+        { effectScope: "#issue-13-scoped-main", nonce: "test-nonce-13" },
+      );
+    }
+  });
+
+  const scopedP = page.locator("#issue-13-scoped-main .scoped-p");
+  const extH2 = page.locator("#issue-13-external-header .ext-h2");
+  const scopedMain = page.locator("#issue-13-scoped-main");
+  const extHeader = page.locator("#issue-13-external-header");
+
+  // Verify scoped paragraph is scaled (16 * 1.25 = 20px)
+  await expect(scopedP).toHaveCSS("font-size", "20px");
+
+  // Verify external header is unaffected
+  await expect(extH2).toHaveCSS("font-size", "16px");
+
+  // Verify scoped container received grayscale filter, while external header did NOT
+  await expect(scopedMain).toHaveCSS("filter", /grayscale\(1\)/);
+  await expect(extHeader).toHaveCSS("filter", "none");
+
+  // Undo session in browser
+  await page.evaluate(() => {
+    const win = window as unknown as {
+      __issue13_session?: { undo: () => void };
+    };
+    if (win.__issue13_session) {
+      win.__issue13_session.undo();
+      delete win.__issue13_session;
+    }
+  });
+
+  // Verify restoration
+  await expect(scopedP).toHaveCSS("font-size", "16px");
+
+  // Clean up host elements
+  await page.evaluate(() => {
+    document.getElementById("issue-13-browser-host")?.remove();
+  });
+});
+
 test("reading guide and mask overlays never block clicks and clean up on Escape and reset", async ({
   page,
 }) => {
diff --git a/tests/scopedEffects.test.tsx b/tests/scopedEffects.test.tsx
new file mode 100644
index 0000000..a519a3d
--- /dev/null
+++ b/tests/scopedEffects.test.tsx
@@ -0,0 +1,256 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import React from "react";
+import { render, screen, fireEvent, cleanup } from "@testing-library/react";
+import {
+  AccessibilityPlugin,
+  AccessibilityButton,
+  AccessibilityList,
+  TextScaleController,
+  ResetController,
+  resolveEffectScope,
+  applyPageSession,
+} from "../src";
+
+describe("Issue #13: Scoped effects, strict CSP, and host compatibility", () => {
+  let container: HTMLDivElement;
+
+  beforeEach(() => {
+    localStorage.clear();
+    sessionStorage.clear();
+    document.body.innerHTML = "";
+    document.documentElement.className = "";
+
+    container = document.createElement("div");
+    container.id = "test-host-root";
+    container.innerHTML = `
+      <header id="external-header" style="position: fixed; top: 0; left: 0; font-size: 16px;">
+        <h1>External Header</h1>
+        <nav><a href="#home">Home</a></nav>
+      </header>
+      <main id="scoped-content" style="margin-top: 60px; font-size: 16px;">
+        <h2 class="content-title">Scoped Main Content</h2>
+        <p class="content-p" style="font-size: 16px; line-height: 24px;">This is scoped content.</p>
+        <a href="#details" class="content-link">Details Link</a>
+      </main>
+      <footer id="external-footer" style="font-size: 14px;">
+        <p>External Footer</p>
+      </footer>
+    `;
+    document.body.appendChild(container);
+  });
+
+  afterEach(() => {
+    cleanup();
+    document.body.innerHTML = "";
+    localStorage.clear();
+    sessionStorage.clear();
+  });
+
+  describe("resolveEffectScope helper", () => {
+    it("resolves document.body when scope is omitted, null, or empty string", () => {
+      expect(resolveEffectScope(undefined)).toBe(document.body);
+      expect(resolveEffectScope(null)).toBe(document.body);
+      expect(resolveEffectScope("")).toBe(document.body);
+    });
+
+    it("resolves CSS selector string to matching HTMLElement", () => {
+      const resolved = resolveEffectScope("#scoped-content");
+      expect(resolved).toBe(document.getElementById("scoped-content"));
+    });
+
+    it("resolves HTMLElement directly", () => {
+      const el = document.getElementById("scoped-content") as HTMLElement;
+      expect(resolveEffectScope(el)).toBe(el);
+    });
+
+    it("resolves React RefObject", () => {
+      const ref = {
+        current: document.getElementById("scoped-content") as HTMLElement,
+      };
+      expect(resolveEffectScope(ref)).toBe(ref.current);
+    });
+
+    it("falls back safely to document.body when selector does not match", () => {
+      expect(resolveEffectScope("#non-existent-selector")).toBe(document.body);
+    });
+  });
+
+  describe("Scoped element traversal and style isolation", () => {
+    it("applies text scaling strictly to scoped container and leaves external elements untouched", async () => {
+      const scopedMain = document.getElementById(
+        "scoped-content",
+      ) as HTMLElement;
+      const externalHeader = document.getElementById(
+        "external-header",
+      ) as HTMLElement;
+      const externalP = externalHeader.querySelector("h1") as HTMLElement;
+      const scopedP = scopedMain.querySelector(".content-p") as HTMLElement;
+
+      render(
+        <AccessibilityPlugin
+          persist={false}
+          effectScope="#scoped-content"
+          position="right"
+        >
+          <AccessibilityButton />
+          <AccessibilityList>
+            <TextScaleController />
+            <ResetController />
+          </AccessibilityList>
+        </AccessibilityPlugin>,
+      );
+
+      // Open panel
+      fireEvent.click(screen.getByRole("button", { name: /accessibility/i }));
+
+      // Increase text size (+12.5%)
+      const increaseBtn = screen.getByLabelText("Increase text size");
+      fireEvent.click(increaseBtn);
+
+      // Scoped paragraph is scaled (16 * 1.125 = 18px)
+      expect(scopedP.style.fontSize).toMatch(/18px/);
+
+      // External elements outside #scoped-content MUST NOT be styled
+      expect(externalP.style.fontSize).toBe("");
+      expect(externalHeader.style.fontSize).toBe("16px");
+
+      // Reset restores scoped paragraph
+      fireEvent.click(screen.getByRole("button", { name: "Reset All" }));
+      expect(scopedP.style.fontSize).toBe("16px");
+    });
+
+    it("scopes CSS filter directly to container, protecting external fixed elements from stacking context bugs", () => {
+      const scopedMain = document.getElementById(
+        "scoped-content",
+      ) as HTMLElement;
+      const externalHeader = document.getElementById(
+        "external-header",
+      ) as HTMLElement;
+
+      // Apply grayscale with effectScope targeting #scoped-content
+      const session = applyPageSession(
+        { grayscale: true },
+        { effectScope: "#scoped-content" },
+      );
+
+      // Scoped container receives filter
+      expect(scopedMain.style.filter).toContain("grayscale(1)");
+
+      // External header (fixed navbar) does NOT receive filter, preserving fixed viewport positioning
+      expect(externalHeader.style.filter).toBe("");
+
+      // Undo cleanly removes filter from scoped container
+      session.undo();
+      expect(scopedMain.style.filter).toBe("");
+    });
+  });
+
+  describe("Incremental mutations in scoped containers", () => {
+    it("styles newly added children inside scope and ignores additions outside scope", () => {
+      const session = applyPageSession(
+        { textScale: 1.2 },
+        { effectScope: "#scoped-content" },
+      );
+
+      const scopedMain = document.getElementById(
+        "scoped-content",
+      ) as HTMLElement;
+      const externalHeader = document.getElementById(
+        "external-header",
+      ) as HTMLElement;
+
+      // Add child to scopedMain
+      const newScopedChild = document.createElement("p");
+      newScopedChild.className = "new-scoped-child";
+      newScopedChild.style.fontSize = "20px";
+      scopedMain.appendChild(newScopedChild);
+
+      // Add child to externalHeader (outside scope)
+      const newExternalChild = document.createElement("p");
+      newExternalChild.className = "new-external-child";
+      newExternalChild.style.fontSize = "20px";
+      externalHeader.appendChild(newExternalChild);
+
+      // Process incremental additions
+      session.applyIncremental([newScopedChild, newExternalChild]);
+
+      // Scoped child is scaled (20 * 1.2 = 24px)
+      expect(newScopedChild.style.fontSize).toBe("24px");
+
+      // External child is ignored and retains original size
+      expect(newExternalChild.style.fontSize).toBe("20px");
+
+      session.undo();
+      expect(newScopedChild.style.fontSize).toBe("20px");
+    });
+  });
+
+  describe("Open Shadow DOM element traversal", () => {
+    it("traverses open shadow roots inside scoped container", () => {
+      const scopedMain = document.getElementById(
+        "scoped-content",
+      ) as HTMLElement;
+
+      // Create a web component with open shadow root
+      const customWidget = document.createElement("div");
+      customWidget.id = "custom-shadow-widget";
+      const shadowRoot = customWidget.attachShadow({ mode: "open" });
+      const shadowP = document.createElement("p");
+      shadowP.className = "shadow-text";
+      shadowP.style.fontSize = "15px";
+      shadowP.textContent = "Shadow DOM Paragraph";
+      shadowRoot.appendChild(shadowP);
+
+      scopedMain.appendChild(customWidget);
+
+      const session = applyPageSession(
+        { textScale: 1.2 },
+        { effectScope: "#scoped-content" },
+      );
+
+      // Element inside open shadow root receives styling
+      expect(shadowP.style.fontSize).toBe("18px"); // 15 * 1.2 = 18px
+
+      session.undo();
+      expect(shadowP.style.fontSize).toBe("15px");
+    });
+  });
+
+  describe("Strict CSP nonce support", () => {
+    it("propagates nonce attribute to injected focus highlight style tag", () => {
+      const testNonce = "csp-nonce-abc-123";
+      const session = applyPageSession(
+        { highlightFocus: true },
+        { nonce: testNonce },
+      );
+
+      const styleEl = document.getElementById(
+        "arap-focus-highlight-style",
+      ) as HTMLStyleElement | null;
+      expect(styleEl).not.toBeNull();
+      expect(styleEl?.nonce).toBe(testNonce);
+      expect(styleEl?.getAttribute("nonce")).toBe(testNonce);
+
+      session.undo();
+      expect(document.getElementById("arap-focus-highlight-style")).toBeNull();
+    });
+
+    it("propagates nonce attribute to injected reduce motion style tag", () => {
+      const testNonce = "csp-nonce-xyz-789";
+      const session = applyPageSession(
+        { reduceMotion: true },
+        { nonce: testNonce },
+      );
+
+      const styleEl = document.getElementById(
+        "arap-reduce-motion-style",
+      ) as HTMLStyleElement | null;
+      expect(styleEl).not.toBeNull();
+      expect(styleEl?.nonce).toBe(testNonce);
+      expect(styleEl?.getAttribute("nonce")).toBe(testNonce);
+
+      session.undo();
+      expect(document.getElementById("arap-reduce-motion-style")).toBeNull();
+    });
+  });
+});