HTML Guide
Backslashes (\) are not allowed in href values; use forward slashes (/) to separate path segments in URLs.
The href attribute in the a (anchor) element defines the hyperlink target and must contain a valid URL. According to the WHATWG HTML Standard, URL paths must use forward slashes (/) as delimiters, not backslashes (\). Backslashes are not recognized by web browsers as valid path separators and will cause validation errors or unexpected behavior. This issue often occurs when copying Windows file paths, which use backslashes, into HTML.
Incorrect HTML:
<a href="folder\page.html">Link</a>Correct HTML:
<a href="folder/page.html">Link</a>If you need to link to a file or resource, always replace any backslashes with forward slashes for proper HTML and browser compatibility.
Learn more:
Related W3C validator issues
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>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.
Spaces in the URL fragment are invalid; encode them or remove them (e.g., use %20 or hyphens/underscores).
The href attribute must contain a valid URL. When using a fragment identifier (the part after #), it must follow URL syntax rules: no unescaped spaces. Fragments usually reference an element’s id. An element’s id must be unique and is case-sensitive; while spaces aren’t allowed in id values, many authors accidentally mirror text with spaces in the fragment. Use hyphens or underscores in ids and match the fragment, or percent-encode reserved characters. Prefer readable, dash-separated ids for accessibility and shareable links.
For example, instead of href=”#My Section”, use href=”#my-section” and set id=”my-section” on the target. If you must preserve spaces in a generated URL, encode them as %20, but it’s better to avoid spaces entirely in ids.
HTML Examples
Invalid: reproduces the validator error
<!doctype html>
<html lang="en">
<head>
<title>Fragment with space</title>
</head>
<body>
<a href="#My Section">Go to section</a>
<h2 id="My Section">My Section</h2>
</body>
</html>Fixed: use a valid fragment and id
<!doctype html>
<html lang="en">
<head>
<title>Valid fragment</title>
</head>
<body>
<a href="#my-section">Go to section</a>
<h2 id="my-section">My Section</h2>
</body>
</html>Alternatively, encoding the space also passes validation, though less ideal as the id would be invalid because it contains spaces:
<a href="#My%20Section">Go to section</a>
<h2 id="My Section">My Section</h2>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.
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.
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://).
Curly braces {} are not allowed in the href attribute value of an <a> element because they are not permitted in valid URLs.
According to the HTML standard and URL specification, certain characters—including { and }—must be percent-encoded in URLs to avoid validation errors and ensure proper browser handling. If you need to include a curly brace in a URL, use percent-encoding: { is %7B and } is %7D.
Incorrect HTML:
<a href="http://example.com/?i=470722{0}">Link</a>Correct HTML with percent-encoding:
<a href="http://example.com/?i=470722%7B0%7D">Link</a>Resulting link:
http://example.com/?i=470722%7B0%7D
Only use plain { or } if you are generating URLs client-side (for example, as template placeholders in JavaScript). To validate properly, always encode or remove illegal characters in attribute values.
A space in the href URL wasn’t percent-encoded, making the link invalid.
The href attribute must contain a valid URL. Spaces and certain characters are not allowed in URLs and must be percent-encoded. In paths and query strings, replace spaces with %20 (or use + only in application/x-www-form-urlencoded query values, not paths).
Prefer generating properly encoded URLs server-side or via encodeURI/encodeURIComponent in JS. For fragment-only links, use href="#id". Avoid unencoded spaces anywhere after the scheme (e.g., https: or mailto:).
HTML Examples
Invalid (reproduces the validator error)
<a href="https://example.com/files/My File.pdf">Download</a>Fixed (spaces percent-encoded)
<a href="https://example.com/files/My%20File.pdf">Download</a>