This criterion was originally adopted to address problems that assistive technology had directly parsing HTML. Assistive technology no longer has any need to directly parse HTML. Consequently, these problems either no longer exist or are addressed by other criteria. This criterion no longer has utility and is removed.
Ensure that HTML or markup code is well-formed and valid, so assistive technologies (like screen readers) can correctly interpret the structure and meaning of content.
Even small coding errors (like missing tags or duplicate IDs) can cause major accessibility issues, making it impossible for users with disabilities to navigate or interact properly.
Key Points
- The document must use valid, properly nested HTML.
- Every start tag has a matching end tag.
- Elements are not overlapped (e.g., one tag shouldn’t close inside another tag it didn’t open in).
- IDs must be unique within the document.
- Ensures assistive technologies can build an accurate accessibility tree.
Short Summary
Invalid or broken HTML confuses browsers and assistive technologies.
Clean, valid code ensures reliable interpretation of structure and meaning — making content stable, predictable, and accessible.
Example Issues and Fixes
Unclosed Tags
<ul>
<li>Home
<li>About
<li>Contact
</ul>
<ul>
<li>Home</li>
<li>About</li>
<li>Contact</li>
</ul>
Overlapping Elements
<p>This is <em>important <strong>text</em></strong>.</p><p>This is <em>important <strong>text</strong></em>.</p>Duplicate IDs
<label for="email">Email</label>
<input id="email" type="email">
<label for="email">Confirm Email</label>
<input id="email" type="email">
<label for="email">Email</label>
<input id="email" type="email">
<label for="confirm-email">Confirm Email</label>
<input id="confirm-email" type="email">
Quick Checklist
- All HTML tags are properly opened and closed.
- Elements are correctly nested (no overlaps).
- All IDs are unique across the page.
- The page validates successfully via W3C Validator.
- ARIA roles and attributes are used correctly and validly.