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Slots let a Web Component display markup supplied by its caller, and Shadow DOM scopes a component’s internal structure and styles. Neither feature sanitizes content. Treat AI-generated text, HTML, CSS, and URLs as untrusted: render plain text as text, and sanitize rich HTML before inserting it.
How slots and Shadow DOM work together
A custom element defines reusable behavior and structure. It can attach a Shadow DOM tree for its internal markup and styles, then use a <slot> as a composition point for content provided by the element’s consumer. MDN describes Shadow DOM as a way to attach a DOM tree to an element while hiding its internals from page JavaScript and CSS. That is useful encapsulation, not a security barrier.
For example, a component’s shadow tree might contain <slot name="heading">Untitled</slot>. A consumer can supply a matching light-DOM child with slot="heading"; the child is rendered at that slot. If no matching child is supplied, the slot’s fallback content, here “Untitled,” can appear. An unnamed slot receives eligible children without a slot attribute. See MDN’s guide to templates and slots.
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Shadow DOM scopes styles: page CSS generally does not style nodes inside the shadow tree, and styles inside it do not style the rest of the page. This reduces accidental selector collisions. It does not make supplied markup safe to parse or render.
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An open shadow root can be accessed through JavaScript, and a closed root should not be treated as a security control. MDN notes that closed roots are not strong protection and may be bypassed, including by browser extensions. Choose open or closed mode based on the component’s access and debugging needs, not as a way to protect untrusted content. See MDN’s Shadow DOM documentation.
Choose a theme interface deliberately
Encapsulation and theming are separate design choices. A component should document which styling hooks it supports—such as host-level CSS custom properties or exposed parts—and which details remain internal. There is no universal theme mechanism established by the platform documentation; select hooks to balance consistent defaults with the customization consumers actually need.
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Slots control composition, not styling safety. Content placed in a slot remains caller-provided content, so do not assume that putting it inside a component makes its text, markup, CSS, or URLs trustworthy.
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Render AI-generated content according to its type
Plain text: insert it as text
If the feature needs only text, set a text node or use textContent rather than parsing a string as HTML. OWASP identifies textContent as a safe basic option for populating the DOM with untrusted data, while emphasizing that safety depends on context. Avoid inserting AI output through innerHTML just to display text.
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Rich HTML: sanitize before insertion
If users or an AI feature are intentionally allowed to supply formatted HTML, define the elements and attributes the feature needs and sanitize against that policy before rendering. MDN documents the HTML Sanitizer API and identifies ShadowRoot.setHTML() as an XSS-safe method for parsing and sanitizing HTML. MDN recommends it over ShadowRoot.innerHTML for untrusted HTML where supported. Check support in the browsers your component targets; if the API is unavailable, use a maintained sanitizer rather than falling back to unsanitized insertion. See MDN’s HTML Sanitizer API guide and MDN’s ShadowRoot.setHTML() reference.
Removing <script> elements alone is not enough. Other markup and attributes can create injection risks, including when inserted through innerHTML. Also avoid modifying sanitized markup afterward in ways that undermine the sanitizer’s protections. OWASP’s guidance covers both XSS prevention and DOM-based XSS prevention.
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CSS and URLs: constrain what the content can control
Keep CSS structure, selectors, and property names under application control. If AI output supplies styling values, accept only values validated for the specific properties your component supports; do not accept arbitrary declarations. Validate URL-bearing values against the destinations and schemes the feature allows. OWASP’s XSS guidance explains these context-specific controls.
Pick the right insertion approach
| Approach | Useful when | Trade-off |
|---|---|---|
Text rendering with textContent |
The feature needs plain text only. | Does not preserve rich formatting, but avoids parsing the string as HTML. |
| Sanitized rich HTML | The feature genuinely needs formatted, caller-supplied markup. | Requires a defined allowlist and a supported Sanitizer API or maintained sanitizer. |
| Shadow DOM | You want scoped internal styles and a defined component boundary. | External styling and access to internals are more constrained; it does not sanitize content. |
| Light DOM | Direct document composition and external styling are priorities. | Component markup and styles are less isolated from the page. |
| Open shadow root | Consumers or debugging tools need access to the root. | Internals are accessible through the root; this is an API choice, not a sanitization measure. |
| Closed shadow root | You want to discourage ordinary page-script access to the root. | It is not dependable security protection and should not be used to secure untrusted content. |
Add defense in depth without relying on it
Content Security Policy (CSP) and Trusted Types can reduce the impact of some injection mistakes when configured for the application. OWASP describes CSP as defense in depth and Trusted Types enforcement for DOM injection sinks in Chromium-based browsers. Neither replaces correct output handling: use text rendering for text and sanitization for rich untrusted HTML. See OWASP’s CSP guidance and its DOM XSS guidance.
Quick Recap
A practical component policy for AI edits
- Decide whether the feature accepts plain text, constrained styling values, or rich HTML; do not silently expand one category into another.
- Render text as text. For rich HTML, define an allowlist and sanitize before insertion.
- Keep CSS selectors and declarations under application control, and validate accepted values and URLs.
- Use slots to define where consumer-provided content appears, not to imply that it has been trusted.
- Document supported theme hooks and choose shadow-root access behavior for API and debugging needs.
- Use CSP and Trusted Types as additional controls where appropriate, not as substitutes for safe rendering.
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