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The href attribute of an <a> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.
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Related W3C validator issues
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.
<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>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”.
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>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 src attribute on an <img> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.
The href attribute on an a tag expects a valid URL, but only http(s):// was found.
The at symbol (@) should be percent-encoded as %40 in order to include it at an href attribute.