HTML Validation

The value of attribute X on element Y from namespace “http://www.w3.org/1999/xhtml” is not in Unicode Normalization Form C.

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About This HTML Issue

Read about Normalization in HTML and CSS.

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→ Normalization in HTML and CSS

Last reviewed: October 27, 2022

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Related W3C Validator Issues

Bad value for attribute “href” on element “a”: Illegal character in scheme data: “\” is not allowed.

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>

Bad value X for attribute “src” on element “script”: Illegal character in path segment: space is not allowed.

The issue arises from the space character in the src attribute value of the script element. In URLs, spaces are not allowed and should be properly encoded to avoid validation errors.

Fix

Replace spaces with %20, which is the URL-encoded representation of a space.

Example

Before:

<script src="https://example.com/media assets/app.js"></script>

After:

<script src="https://example.com/media%20assets/app.js"></script>

Explanation

In this example, the space between “media” and “assets” in the URL is replaced with %20. This change ensures that the URL conforms to standards and is correctly processed by browsers and servers. Spaces and other special characters in URLs must be encoded to ensure proper formatting and accessibility.

Bad value X for attribute “href” on element “a”: Illegal character in path segment

Bad value for attribute “href” on element “a”: Illegal character in query: “[” is not allowed.

Square brackets ([, ]) are not allowed unescaped in the query part of an href URL value.

The href attribute in the <a> element must contain a valid URL. According to the URL standard, certain characters, including square brackets, are not permitted directly in the query component unless percent-encoded. Using unescaped square brackets in the URL can cause validation errors and unexpected behavior in browsers.

To include a square bracket in the query string, use percent encoding:

[ encodes to %5B
] encodes to %5D

Incorrect usage:

<a href="search.html?q=[value]">Search</a>

Correct usage:

<a href="search.html?q=%5Bvalue%5D">Search</a>

This ensures the URL is valid and compliant with HTML standards.

Bad value “X” for attribute “href” on element “link”: Illegal character in path segment: “X” is not allowed.

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.

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