HTML Validation

Bad value for attribute “src” on element “img”: Illegal character in scheme data: “<” is not allowed.

About This HTML Issue

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

Learn more:

→ Wikipedia: percent-encoding

Last reviewed: October 27, 2022

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Related W3C Validator Issues (122)

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

Illegal character “[” in the iframe src URL requires percent-encoding or removal.

The iframe element’s src must be a valid URL. According to URL syntax, characters like [ and ] are not allowed in the query unless percent-encoded. If your src contains array-like parameters (e.g., filters[category]=news), encode reserved characters: [ becomes %5B and ] becomes %5D. Avoid spaces and encode other reserved characters as needed. Alternatively, adjust the server to accept dot or bracketless notation (e.g., filters.category=news or filters_category=news) so the URL stays valid without encoding.

HTML Examples

Example causing the validator error

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Iframe URL Error</title>
</head>
<body>
<!-- [ and ] are illegal in URLs unless encoded -->

  <iframe src="https://example.com/embed?filters[category]=news&filters[tags]=web"></iframe>
</body>
</html>

Fixed example with percent-encoding

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Iframe URL Fixed</title>
</head>
<body>
<!-- Encode [ as %5B and ] as %5D -->

  <iframe src="https://example.com/embed?filters%5Bcategory%5D=news&filters%5Btags%5D=web"></iframe>
</body>
</html>

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

Square brackets in an img src query string must be percent-encoded to be valid.

The src attribute on img must be a valid URL. In URL query strings, characters like [ and ] are not allowed unescaped per URL syntax. When present (often from frameworks adding array-like params), they must be percent-encoded as [ -> %5B and ] -> %5D. Alternatively, remove brackets from the query or use a server-side/route format that avoids them.

HTML examples

Example causing the validator error

<img src="/images/photo.jpg?size[width]=300&size[height]=200" alt="Sample">

Fixed example using percent-encoding

<img src="/images/photo.jpg?size%5Bwidth%5D=300&size%5Bheight%5D=200" alt="Sample">

Fixed example by avoiding brackets in params

<img src="/images/photo.jpg?size_width=300&size_height=200" alt="Sample">

Bad value X for attribute “src” on element “img”: Backslash ("\") used as path segment delimiter.

This error message indicates that there is a backslash (\) used in a URL, which is not a valid character for URL paths.

You’ll need to replace the backslashes with forward slashes (/) in the URL path segments.

Here’s an example of a correct img tag using a valid URL path:

<img src="https://example.com/img/small/photo.png" alt="example image">

Also, make sure that the URL is correct and that the image file actually exists in the specified location.

Bad value X for attribute “src” on element “img”: Illegal character in path segment.

The src attribute on an element <img> contains a character which is not allowed unless properly encoded.

Special characters needing encoding are: :, /, ?, #, [, ], @, !, $, &, ', (, ), *, +, ,, ;, =, as well as % itself.

For example, this image tag is incorrect because the src attribute contains an the unallowed characters [ and ]:

<img src="image[00].svg" alt="logo">

Instead, this is the properly percent-encoded src attribute, where [ has been replaced with %5B and ] with %5D.

<img src="image%5B00%5D.svg" alt="logo">

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

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

<img src="https://example.com/?s=some term" alt="description" />
<img src="https://example.com/?s=some%20term" alt="description" />

Bad value “X” for attribute “src” on element “img”: Illegal character in scheme data: space is not allowed.

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 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>

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