The “complementary” role is unnecessary for element “aside”.

The issue arises because “complimentary” is not a valid value for the role attribute on the <aside> element. In HTML, the role attribute is used to provide assistive technologies with extra information about the purpose of the element. The role values are defined by the WAI-ARIA specification.

The correct role for an <aside> element, if explicitly needed, would be complementary (note the spelling with an “e” instead of an “i”). However, the <aside> element has an implicit ARIA role of complementary, and thus it is typically unnecessary to explicitly specify this role unless you are using advanced ARIA techniques.

Correct Example:

<aside>
  <h2>Related Information</h2>
  <p>Here you can find additional resources and links.</p>
</aside>

If you want to explicitly define the role:

<aside role="complementary">
  <h2>Related Information</h2>
  <p>Here you can find additional resources and links.</p>
</aside>

Make sure to always use correct ARIA attributes and values to maintain accessibility for users relying on assistive technologies. For a comprehensive list of roles available for usage, refer to the WAI-ARIA specifications.

The role attribute exposes an element’s purpose to assistive technologies. ARIA defines a fixed set of role values; sidebar is not among them, so validators report a bad value. Sidebars typically contain related or ancillary content, which maps to the complementary landmark role. In HTML, the <aside> element already represents this concept and implicitly maps to the complementary role.

Leaving an invalid role harms accessibility because screen readers may ignore the landmark or misreport it, and automated tools can’t build a reliable landmarks map. Standards compliance also matters for consistent behavior across browsers and assistive tech.

To fix it:

• Replace role="sidebar" with role="complementary" on a generic container; add an accessible name with aria-labelledby or aria-label when multiple complementary regions exist.
• Prefer <aside> for semantic HTML. It implicitly has the complementary role; add a label when there is more than one <aside>.
• Do not add role="complementary" to <aside> unless you need to override something; duplicate roles are unnecessary.
• If the area is site-wide navigation, use <nav> or role="navigation" instead; choose the role that best matches the intent.

Examples

Invalid: non-existent ARIA role

<div role="sidebar">
<!-- Related links and promos -->

</div>

Fixed: use the complementary role on a generic container

<div role="complementary" aria-labelledby="sidebar-title">
  <h2 id="sidebar-title">Related</h2>
  <ul>
    <li><a href="/guide-a">Guide A</a></li>
    <li><a href="/guide-b">Guide B</a></li>
  </ul>
</div>

Fixed: use semantic HTML with aside (implicit complementary)

<aside aria-labelledby="sidebar-title">
  <h2 id="sidebar-title">Related</h2>
  <ul>
    <li><a href="/guide-a">Guide A</a></li>
    <li><a href="/guide-b">Guide B</a></li>
  </ul>
</aside>

Multiple sidebars: ensure unique, descriptive labels

<aside aria-labelledby="filters-title">
  <h2 id="filters-title">Filter results</h2>
<!-- filter controls -->

</aside>

<aside aria-labelledby="related-title">
  <h2 id="related-title">Related articles</h2>
<!-- related links -->

</aside>

When it’s actually navigation: use the navigation landmark

<nav aria-label="Section navigation">
  <ul>
    <li><a href="#intro">Intro</a></li>
    <li><a href="#examples">Examples</a></li>
    <li><a href="#contact">Contact</a></li>
  </ul>
</nav>

Tips:

• Use <aside> for tangential content; it’s the simplest, standards-based approach.
• Provide an accessible name when more than one complementary region is present.
• Avoid inventing ARIA roles; stick to defined values like banner, main, navigation, complementary, contentinfo, and others.

ARIA defines a fixed set of role values that user agents and assistive technologies understand. sidebar is not in that set, so role="sidebar" fails conformance checking and gives unreliable signals to screen readers. Using a valid role or the correct HTML element improves accessibility, ensures consistent behavior across browsers and AT, and keeps your markup standards‑compliant.

Sidebars typically contain tangential or ancillary content (e.g., related links, promos, author info). The ARIA role that matches that meaning is complementary. In HTML, the semantic element for the same concept is aside, which by default maps to the complementary landmark in accessibility APIs. Prefer native semantics first: use <aside> when possible. Only add role="complementary" when you can’t change the element type or when you need an explicit landmark for non-semantic containers.

How to fix:

• If the element is a sidebar: change <div role="sidebar"> to <aside> (preferred), or to <div role="complementary">.
• Ensure each page has at most one primary main region and that complementary regions are not essential to understanding the main content.
• Provide an accessible name for the complementary region when multiple exist, using aria-label or aria-labelledby, to help users navigate landmarks.

Examples

Triggers the validator error

<div role="sidebar">
<!-- Sidebar content -->

</div>

Fixed: use the semantic element (preferred)

<aside aria-label="Related articles">
<!-- Sidebar content -->

</aside>

Fixed: keep the container, apply a valid role

<div role="complementary" aria-label="Related articles">
<!-- Sidebar content -->

</div>

Full document example with two sidebars (each labeled)

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Sidebar Landmarks Example</title>
  </head>
  <body>
    <header>
      <h1>News Today</h1>
    </header>

    <main id="main">
      <article>
        <h2>Main Story</h2>
        <p>...</p>
      </article>
    </main>

    <aside aria-label="Trending topics">
      <ul>
        <li>Science</li>
        <li>Politics</li>
        <li>Sports</li>
      </ul>
    </aside>

    <div role="complementary" aria-labelledby="sponsor-title">
      <h2 id="sponsor-title">Sponsored</h2>
      <p>Ad content</p>
    </div>

    <footer>
      <p>&copy; 2026</p>
    </footer>
  </body>
</html>

Notes:

• Do not invent ARIA roles (e.g., sidebar, hero, footer-nav). Use defined roles like complementary, navigation, banner, contentinfo, and main.
• Prefer native HTML elements (aside, nav, header, footer, main) over generic containers with roles.
• Label multiple complementary landmarks to make them distinguishable in screen reader landmark lists.

There can only be one visible <main> element in a document. If more are needed (for example for switching between them with JavaScript), only one can be visible, the others should be hidden toggling the hidden attribute.

Example of 2 main elements, where only one is visible:

<main>
  <h1>Active main element</h1>
<!-- content -->

</main>

<main hidden>
  <h1>Hidden main element</h1>
<!-- content -->

</main>

The HTML <figure> element is used to encapsulate media content, such as an image or graphic, along with a descriptive <figcaption>. When a <figcaption> is present within a <figure>, it inherently provides the semantics of the <figure>, making it self-explanatory without needing an additional role attribute.

Explanation

• <figure> element: Represents self-contained content, potentially with an optional caption specified by a <figcaption> element. This is inherently recognized for its semantics as a figure with a caption.

• <figcaption> element: Provides a caption or description for the content of the <figure>. This helps in describing the media or content included in the <figure> element.

• role attribute: This attribute is used to define an explicit accessibility role for an element. However, in cases where the element’s native semantics are explicit and sufficient, such as a <figure> with a <figcaption>, adding a role attribute might override or conflict with the inherent meaning.

Solution

Remove the role attribute from the <figure> element when it contains a <figcaption>.

Example of Incorrect Code:

<figure role="figure">
  <img src="cat.jpg" alt="A cute cat">
  <figcaption>A cute cat looking at the camera.</figcaption>
</figure>

Corrected Code:

<figure>
  <img src="cat.jpg" alt="A cute cat">
  <figcaption>A cute cat looking at the camera.</figcaption>
</figure>

In the corrected example, the <figure> element does not have a role attribute, allowing it to maintain its inherent semantic value.

A role="cell" element must be a child of an element with role="row" for correct ARIA relationships.

According to the ARIA specification, role="cell" should be directly contained within a parent with role="row", which itself should usually be inside an element with role="rowgroup" or role="table". This structure allows assistive technologies to interpret your table semantics correctly.

Correct Structure Example:

<div role="table">
  <div role="row">
    <div role="cell">Row 1, Cell 1</div>
    <div role="cell">Row 1, Cell 2</div>
  </div>
  <div role="row">
    <div role="cell">Row 2, Cell 1</div>
    <div role="cell">Row 2, Cell 2</div>
  </div>
</div>

Incorrect Structure Example (missing row):

<div role="table">
  <div role="cell">Cell without row</div>
</div>

How to fix:
Wrap any element with role="cell" inside an element with role="row". This ensures both validity and proper accessibility support.

A role="columnheader" element must be a child of or associated with a role="row" element.

In HTML, ARIA roles such as columnheader are used to improve accessibility for assistive technologies. According to the ARIA specification, a columnheader role should appear inside an element with role="row", which itself should be inside an element with role="table" or role="grid". This structure mimics how native tables are constructed with <th> elements inside <tr>s.

Correct structure:

• role="table" or role="grid" contains one or more elements with role="row".
• Each role="row" contains one or more elements with role="columnheader" (or role="cell").

Example using ARIA roles for a simple table:

<div role="table" aria-label="Sample Table">
  <div role="row">
    <div role="columnheader">Name</div>
    <div role="columnheader">Age</div>
  </div>
  <div role="row">
    <div role="cell">Alice</div>
    <div role="cell">30</div>
  </div>
</div>

Best practice:
Whenever possible, use native table elements, which have built-in roles and accessibility, reducing the chance of ARIA misuse.

Example using native table markup:

<table>
  <tr>
    <th>Name</th>
    <th>Age</th>
  </tr>
  <tr>
    <td>Alice</td>
    <td>30</td>
  </tr>
</table>

Ensure that any element with role="columnheader" is always contained within a parent with role="row". Avoid placing role="columnheader" directly inside a container without the appropriate role="row" ancestor.

To fix the W3C HTML Validator issue stating that an element with a role="menuitem" must be contained in, or owned by, an element with role="menubar" or role="menu", you need to ensure that your menuitem elements are properly nested within a menubar or menu element. This is important for accessibility, as it helps assistive technologies understand the structure and relationship of the elements.

The menuitem role indicates the element is an option in a set of choices contained by a menu or menubar.

Here is a step-by-step guide to fixing this issue:

1. Using role="menubar"

If your menuitem elements are part of a horizontal menu (like a navigation bar), they should be nested within an element with role="menubar".

Example:

<nav role="menubar">
  <div role="menuitem">Home</div>
  <div role="menuitem">About</div>
  <div role="menuitem">Contact</div>
</nav>

2. Using role="menu"

If your menuitem elements are part of a submenu or a vertical menu, they should be contained within an element with role="menu".

Example:

<div role="menu">
  <div role="menuitem">Item 1</div>
  <div role="menuitem">Item 2</div>
  <div role="menuitem">Item 3</div>
</div>

Ensuring Proper Nesting

Ensure that all your menuitem elements are either directly or indirectly (via a child-parent relationship) contained within a menubar or menu element.

Complete Example with Nested Menus:

Here is a more complex example, including nested menus for a drop-down scenario.

Example:

<nav role="menubar">
  <div role="menuitem">Home</div>
  <div role="menuitem">
    About
    <div role="menu">
      <div role="menuitem">Team</div>
      <div role="menuitem">History</div>
    </div>
  </div>
  <div role="menuitem">Contact</div>
</nav>

In this example, the main navigation (menubar) contains menuitem elements, and one of those menuitem elements contains a nested menu with additional menuitem elements inside it.

By ensuring your menuitem elements are contained within appropriate parent elements (menubar or menu), you will resolve the W3C HTML Validator issue and improve your web page’s accessibility.

Elements with the role tab must either be a child of an element with the tablist role, or have their id part of the aria-owns property of a tablist.

An element with the tab role controls the visibility of an associated element with the tabpanel role. The common user experience pattern is a group of visual tabs above, or to the side of, a content area, and selecting a different tab changes the content and makes the selected tab more prominent than the other tabs.

Example:

<div class="tabs">
  <div role="tablist" aria-label="Sample Tabs">
    <button role="tab" aria-selected="true" aria-controls="panel-1" id="tab-1" tabindex="0">
          First Tab
    </button>
    <button role="tab" aria-selected="false" aria-controls="panel-2" id="tab-2" tabindex="-1">
          Second Tab
     </button>
  </div>
  <div id="panel-1" role="tabpanel" tabindex="0" aria-labelledby="tab-1">
    <p>Content for the first panel</p>
  </div>
  <div id="panel-2" role="tabpanel" tabindex="0" aria-labelledby="tab-2" hidden>
    <p>Content for the second panel</p>
  </div>
</div>

A button element, or an element with the role=button attribute, is not allowed to be nested inside an <a> element.