HTML Guide for a

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

An <a> element cannot contain a descendant element with the attribute tabindex.

The href attribute of an <a> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.

The href attribute of the a element contains an invalid backslash character, which is not permitted in URLs.

According to the WHATWG HTML living standard, the href attribute must contain a valid URL. URLs use forward slashes (/) for path separators, and backslashes are not allowed as they can cause browsers and validators to misinterpret the address. Backslashes often arise when file paths are copied from Windows environments.

Correct Usage:

• Always use forward slashes / in your URLs.
• Remove any backslashes from href values.

Example of incorrect usage:

<a href="images\picture.jpg">View Picture</a>

Corrected example:

<a href="images/picture.jpg">View Picture</a>

The target attribute on <a> elements can’t be blank.

This attribute defines the browsing context for links, that is, where should the linked documents be opened. This was used extensively on the now deprecated <frame> element, so you could give the name of the frame to open the document in, but is now more used to force links to open in a separate tab or window using target="_blank". Another option is using a name, so the new browsing context can be referred to on subsequent clicks on links with the same target.

For example, this will force the links to open on a new tab:

<a href="https://example.com" target="_blank">will open a blank tab</a>

The href attribute on an a tag expects a valid URL, but only http(s):// was found.

<a> tags can be used to link to an email address using the mailto protocol in the href attribute. Ensure that there is no space in the email address.

<a href="mailto: liza@example.com">This is wrong as it contains an space</a>

<a href="mailto:liza@example.com">This is OK</a>

The W3C HTML Validator issue you encountered indicates that the URL provided in the href attribute of an anchor (<a>) element is not formatted correctly.

How to Fix the Issue

• Check the Protocol: For a valid URL, make sure that after https: there are two slashes (//).
• Update the URL: Correct the URL format to include the missing slash.

Example of Incorrect HTML

Here is an example of the code that would trigger the validation error:

<a href="https:/example.comf">Example</a>

Corrected HTML

Here’s how the corrected code should look:

<a href="https://example.com">Example</a>

Summary

Make sure that all URLs within href attributes are correctly formatted with both slashes following the protocol (https:// or http://).

Hash (#) characters can be used in an href attribute to link to a specific part of a document.

For example, if we have this page with several sections, each of them marked with an ID:

<h1>Frequently Asked Questions</h1>

<h2 id="pricing">Pricing</h2>
<p>All about pricing...</p>

<h2 id="terms">Terms</h2>
<p>You can find our terms at...</p>

<h2 id="guarantee">Guarantee</h2>
<p>We offer a guarantee...</p>

You can link to a specific part of that document, for example if this page URL is /faqs and you want to link to the Guarantee section you could use:

<a href="/faqs#guarantee">Guarantee</a>

Or, if you’re linking from inside the same document, for example in a table of contents, you could just use:

<a href="#guarantee">Guarantee</a>

As there can only be one fragment in an URL, the # character should only be used once. The following would be an invalid href:

<a href="/faqs#guarantee#pricing">Bad</a>

If needed, the # could be encoded as %23.

Space characters are not allowed in href attributes. Instead, they should be converted to %20. In this example, the first line is invalid and the second is valid:

<a href="https://example.com#some term">invalid</a>
<a href="https://example.com#some%20term">valid</a>

The href attribute on an <a> tag contains an space, which is not allowed. Consider replacing space characters with “%20”.

The attribute value in the href on the <a> element contains invalid characters.

HTML links require a valid URL as the value of the href attribute. Brackets ([, ]) are not allowed in attribute values.

Example of invalid usage:

<a href="mailto:[user@example.com]">Email Us</a>

Correct usage:

<a href="mailto:user@example.com">Email Us</a>

If you are using a template variable to create the links, be sure the template engine properly outputs a valid address in the final HTML.

Example using pseudocode for a templating engine (replace with your syntax as needed):

<a href="mailto:{{ address }}">Email Us</a>

Correct, fully rendered HTML after the template variable is replaced:

<a href="mailto:user@example.com">Email Us</a>

An href attribute on an a element contains an invalid URL that has space characters in the domain.

The domain in a URL cannot contain space characters, for example the following are invalid:

<a href="http://my domain.com">link</a>
<a href="http://my%20domain.com">link</a>

The W3C Validator issue you’re seeing is because the ping attribute on the <a> (anchor) element is supposed to contain a space-separated list of URLs to which, when the hyperlink is activated, the browser will send ping requests. The ping attribute must only contain proper “http” or “https” URLs, that is, relative URLs are not allowed.

Example of Incorrect Code

<a href="https://example.com" ping="/track">Visit Example</a>

Examples of Correct Code

1. Proper Ping URL: Ensure the ping URL starts with http or https and is correctly formatted.

   <a href="https://example.com" ping="https://example.com/track">Visit Example</a>

2. Multiple URLs: If you have multiple URLs, they should be space-separated and each should be a valid URL.

   <a href="https://example.com" ping="https://example.com/track https://example.com/another-track">Visit Example</a>

Common Mistakes to Avoid

• Using invalid URL schemes like mailto: or /relative-path.
• Including broken URLs or URLs with protocols other than http or https.
• Incorrectly formatting multiple URLs; they must be space-separated, not comma-separated or otherwise.

Note

The ping property is not effective in Firefox and its usage may be limited due to privacy and security concerns.

The value used in the target property of an <a> element has been identified as a keyword because it starts with an underscore _, but it’s not a valid keyword.

Values starting with an underscore for the target property are used for reserved keywords like _blank, _self, _parent or _top.

The value provided on the type attribute of an a element is not a valid MIME type.

The type attribute expects a MIME type that hints at the linked URL’s format.

The type attribute on <a> elements, when present, gives a hint on the MIME type of the linked resource, for example:

<a href="application/pdf" src="book.pdf">Read our book</a>
<a href="image/jpeg" src="photo.jpeg">See a photo</a>

That is, we’re talking about the type of the linked resource, not the type of the <a> element, as it’s sometimes misunderstood. The following example is invalid because button is not a valid MIME type.

<a href="/order.php" type="button">Submit</a>

The <a> element requires either a href attribute, or a role attribute.

An </a> end tag has been found to violate nesting rules. <a> tags can’t include other <a> tags inside. Most probable cause is an unclosed <a> tag, like in this example:

<a href="one.html">Page 1
<a href="two.html">Page 2</a>

An <a> tag can’t include other <a> tags inside. Most probable cause is an unclosed <a> tag, like in this example:

<a href="one.html">Page 1
<a href="two.html">Page 2</a>

An <a> element has been found in an invalid place within a <table> element.

For example, the following code would cause this issue:

<table>
  <tr>
    <a href="#">link</a>
  </tr>
</table>

Instead, the <a> element should be inside a <td> element, as a <tr> can’t hold content directly:

<table>
  <tr>
    <td>
      <a href="#">link</a>
    </td>
  </tr>
</table>

When you encounter the issue where the <a> element is nested inside an element with role="button", it’s important to understand that this can lead to accessibility conflicts. The role="button" indicates that the element is interactive, similar to a button. Nesting an <a> (which is also an interactive element) inside it can confuse assistive technologies.

How to Fix This Issue

You should either change the structure so that the <a> is not inside the button or change the role of the button to avoid this violation. Here are two common approaches to resolve the issue:

Option 1: Remove the <a> Tag

Replace the <a> tag with an appropriate action directly inside the element with role="button".

Example Before:

<div role="button">
  <a href="#link">Click here</a>
</div>

Example After:

<div role="button" tabindex="0" onclick="location.href='#link';">
  Click here
</div>

Here, we use JavaScript to navigate to the link when the div is clicked.

Option 2: Remove the role="button"

If the <a> tag is sufficient by itself, you can remove the role="button" from the surrounding element.

Example Before:

<div role="button">
  <a href="#link">Click here</a>
</div>

Example After:

<a href="#link">Click here</a>

This maintains the desired navigation without creating a conflict between the button and link semantics.

An <a> element (anchor) cannot be nested inside another <a> element.

Nesting anchor elements is invalid HTML according to the W3C and WHATWG specifications. Only phrasing content (such as text, images, or spans) is allowed inside an <a>. Placing an <a> inside another <a> can confuse assistive technology and browsers, leading to unpredictable behavior and accessibility issues.

Incorrect example:

<a href="https://example.com">
  Visit <a href="https://another.com">Another Site</a>
</a>

Correct usage: Separate the anchor elements so they do not overlap.

<p>
  Visit <a href="https://example.com">Example Site</a> 
  and 
  <a href="https://another.com">Another Site</a>
</p>

Full minimal HTML example:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>No Nested Anchors</title>
  </head>
  <body>
    <p>
      Check <a href="https://site1.com">Site 1</a> or 
      <a href="https://site2.com">Site 2</a> for more information.
    </p>
  </body>
</html>

Always ensure anchor tags are not placed inside other anchor tags to maintain valid and accessible HTML.

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

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