The Native Sanitizer API

Technical Deep Dive & Specifications

The Safe-by-Default Baseline

By default, calling element.setHTML(dirtyString) automatically applies a hardened, secure baseline defined by the WHATWG and W3C specifications:

1. Executable Elements Stripped: <script>, <object>, <embed>, <iframe>, <applet>, <frame>, <frameset>.
2. Inline Event Handlers Stripped: onclick, onerror, onload, onmouseover, and all other on* attributes.
3. Malicious Protocols Blocked: javascript: URIs inside href or src attributes are sanitized and removed.
4. Dangerous Metadata Blocked: <meta http-equiv>, <base>, <link rel="import">.

       UNTRUSTED INPUT                                       SAFE NATIVE DOM
+------------------------------------+          +------------------------------------+
| <p>Hello <script>alert(1)</script> |          | <p>                                |
|    <b onclick="steal()">Click</b>  | ======>  |   Hello                            |
|    <a href="javascript:hack()">Link|          |   <b>Click</b>                     |
| </p>                               |          |   <a>Link</a>                      |
+------------------------------------+          +------------------------------------+

Customizing the Sanitizer Configuration

You can tailor the sanitization policy by passing a configuration object to the Sanitizer constructor or directly to setHTML():

// Define custom sanitization policy
const customSanitizer = new Sanitizer({
  // Only permit safe typography and hyperlinks
  elements: ['p', 'b', 'strong', 'em', 'i', 'a', 'ul', 'ol', 'li', 'code', 'pre'],
  
  // Explicitly strip styling and tracking tags
  removeElements: ['style', 'font', 'marquee', 'blink'],
  
  // Permit safe attributes
  attributes: ['href', 'title', 'class', 'alt'],
  
  // Strip inline styles and custom event data
  removeAttributes: ['style', 'id']
});

// Apply policy to target container
targetElement.setHTML(untrustedUserMarkdownHtml, { sanitizer: customSanitizer });

Comparison: innerHTML vs. DOMPurify vs. Native setHTML()

Feature Detection and Progressive Fallback

Because the Sanitizer API is rolling out across evergreen browser engines, production applications should employ progressive feature detection:

function setSafeHTML(element, untrustedMarkup, config = {}) {
  if ('setHTML' in Element.prototype) {
    // Native Living Standard Sanitizer API
    element.setHTML(untrustedMarkup, config);
  } else {
    // Fallback: Use DOMPurify or textContent if unavailable
    console.warn('Native Sanitizer API unavailable; using fallback.');
    if (window.DOMPurify) {
      element.innerHTML = DOMPurify.sanitize(untrustedMarkup);
    } else {
      element.textContent = untrustedMarkup; // Fallback to safe plain text
    }
  }
}

💻 Interactive Code Playground

Starter Code: Production XSS Defense Lab

Line-by-Line Code Breakdown

• Lines 63–68: Pre-populates the input with 4 real-world attack vectors: an onerror attribute, an inline javascript: link, an executable <script> tag, and valid formatted text.
• Lines 89–93: Calls rendered.setHTML(dirtyHtml). The browser parser processes the stream, immediately stripping the <script> tag, nullifying the onerror attribute, and cleaning the malicious javascript: URL without raising any alerts.
• Lines 94–112: Provides a safe fallback loop for browsers where the native flag has not yet been toggled on by default.

Expected Browser Render Output

(Notice: All alert() scripts, onerror listeners, and javascript: URLs were neutralized automatically.)

Live Rendered Output:
Welcome, Alex!
[Broken Image Icon]
Click Free Gift

Sanitized HTML Source Tree:
<p>Welcome, <strong>Alex</strong>!</p>
<img src="invalid-image">
<a>Click Free Gift</a>