HTML Guide

Spaces are not permitted in the href value for phone links; the phone number must be a continuous string without spaces or slashes.

The href attribute of an anchor (<a>) element defines the link’s destination. For phone numbers, the proper URI scheme is tel:, not callto:. According to the HTML standard and the WHATWG Living Standard, the phone number should contain only digits and may use plus (+) or hyphen (-) characters for formatting, but it should not include spaces or slashes.

Incorrect HTML:

<a href="callto:07142/ 12 34 5">Call us</a>

Correct HTML:

<a href="tel:0714212345">Call us</a>

With country code and optional formatting:

<a href="tel:+49714212345">Call us</a>

For best compatibility and validation, always use the tel: scheme and ensure the phone number string contains only allowed characters.

An <a> element has been found with an invalid href attribute, containing more than one # adjacent character.

The # is used to separate the fragment part of an URI (typically used to indicate a section within a document). For example, this is a valid link to a URI containing a fragment:

<a href="https://example.com/faqs#pricing">pricing</a>

The next example is invalid because it contains two adjacent # characters, so that the fragment part would be #pricing instead of pricing:

<a href="https://example.com/faqs##pricing">pricing</a>

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

The pipe character | is not permitted in the query component of a URL in the href attribute of an a element.

According to the WHATWG and W3C HTML specifications, URLs in attributes such as href must be valid and properly encoded. The pipe character | is not a valid character in the query string of a URL unless it is percent-encoded as %7C. Failing to encode it will cause validation errors. This is especially important for interoperability and security across browsers and user agents.

Incorrect example (invalid href with pipe):

<a href="https://example.com/search?q=test|demo">Invalid link</a>

Correct example (pipe character encoded):

<a href="https://example.com/search?q=test%7Cdemo">Valid link</a>

Always encode special characters such as | in URLs used within HTML attributes to ensure your documents validate and behave consistently.

Space characters are not permitted in the value of the href attribute; they must be properly percent-encoded.

The href attribute specifies a URL, and URLs must follow specific syntax rules defined by RFC 3986. Spaces and some other characters are considered illegal in URLs. To include a space in the URL, use the percent escape sequence %20 in place of the space character.

Incorrect example with an illegal space in the query string:

<a href="search.html?q=my search">Search for 'my search'</a>

Correct example using percent-encoding for the space:

<a href="search.html?q=my%20search">Search for 'my search'</a>

Replace all spaces in URLs within href attributes with %20 to ensure W3C validation and proper browser behavior.

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>

<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://).

An illegal character has been found for the “href” attribute on the “link” element.

To fix this issue, find the “link” element in question and make sure that the “href” attribute contains a valid URL without any illegal characters.

Here’s some example HTML code of a link element:

<!DOCTYPE html>
<html>
  <head>
    <title>My Webpage</title>
    <link rel="stylesheet" href="styles/main.css">
  </head>
  <body>
    <h1>Welcome to my webpage!</h1>
    <p>Here is some content...</p>
  </body>
</html>

In the above example, the link element has a valid href attribute value of styles/main.css. Make sure that your href attribute values don’t contain any illegal characters.

Empty type attributes are invalid on the <a> element; you must either remove the type attribute or provide a valid MIME type value.

The type attribute on an <a> (anchor) element specifies the MIME type of the linked resource. According to the WHATWG HTML specification and W3C validator, the value of the type attribute cannot be an empty string. It should be a valid MIME type, such as application/pdf or text/html. If you do not know the MIME type or it’s not relevant, you should omit the attribute.

Incorrect HTML example (causes validation error):

<a href="document.pdf" type="">Download PDF</a>

Correct HTML example (remove the invalid attribute):

<a href="document.pdf">Download PDF</a>

Correct HTML example (provide a valid MIME type):

<a href="document.pdf" type="application/pdf">Download PDF</a>

If the type is unknown or unnecessary, leaving it out is preferred to using an empty value.