HTML Guide
The href attribute on an element <link> contains a character that is not allowed, and should be encoded.
Some typical examples include the pipe character | that should be replaced by its encoded alternative %7C , and the left square bracket [ that needs to be encoded as %5B.
Related W3C validator issues
A link element is invalid unless it contains at least one of the href or imagesrcset attributes.
The link element is used to define relationships between the current document and external resources, most commonly stylesheets. According to the HTML standard, a link must have either an href attribute, which points to the resource, or an imagesrcset attribute for responsive icons. If both are missing, the validator throws an error. This ensures that the link actually references a resource, otherwise it serves no functional purpose.
Correct usage:
With an href (for stylesheets, icons, etc.)
<link rel="stylesheet" href="styles.css">With imagesrcset (for responsive icons)
<link rel="preload" as="image" type="image/png" imagesrcset="icon-1x.png 1x, icon-2x.png 2x">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.
The href attribute on the link element must not be empty.
The href attribute on an element <a> contains a character that is not allowed, and should be encoded.
Some typical examples include the pipe character | that should be replaced by its encoded alternative %7C , and the left square bracket [ that needs to be encoded as %5B.
An a element with an href attribute provides a link to a resource, so adding the link role to it is redundant.
When not using semantic HTML for its intended purpose, interactive features must be re-implemented. For example, when role="link" is added to an element, the tab key should enable giving focus to the link and the enter key should execute the link when focused.
A <link> element has been found in an invalid body context. Check the attributes of the <link> element and ensure it’s not within the <body> section.
If the element is within the <head> section, it may have been interpreted as a body context depending on previous elements. For example, while this <link> element is valid per se and is in the <head> section, it is deemed invalid because the previous <img> element made the validator consider it a body context:
<!DOCTYPE html>
<html lang="">
<head>
<title>Test</title>
<img src="photo.jpg" alt="A smiling cat" />
<link rel="canonical" href="https://example.com/" />
</head>
<body>
<p>Some content</p>
</body>
</html>If we fix that document and move the <img> tag within the body, the issue raised about <meta> disappears because it’s now in a valid context:
<!DOCTYPE html>
<html lang="">
<head>
<title>Test</title>
<link rel="canonical" href="https://example.com/" />
</head>
<body>
<p>Some content</p>
<img src="photo.jpg" alt="A smiling cat" />
</body>
</html>A <link> element that is using the preload value in the rel attribute is missing the as attribute, used to indicate the type of the resource.
The preload value of the <link> element’s rel attribute lets you declare fetch requests in the HTML’s <head>, specifying resources that your page will need very soon, which you want to start loading early in the page lifecycle, before browsers’ main rendering machinery kicks in. This ensures they are available earlier and are less likely to block the page’s render, improving performance.
The as attribute specifies the type of content being loaded by the <link>, which is necessary for request matching, application of correct content security policy, and setting of correct Accept request header.
The error message indicates that the <link> element contains an as attribute, but its rel attribute does not hold the correct value. According to the HTML specification, when using the as attribute, the rel attribute must be set to "preload" or "modulepreload".
Explanation
The as attribute is used to specify the type of content being loaded, which helps the browser to handle it appropriately. To preload resources, you need to specify the rel attribute with the value "preload" (for general resources) or "modulepreload" (specifically for JavaScript modules).
Example with preload:
If you are preloading a stylesheet, the rel attribute should be set to "preload":
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Correct Usage of Link with as Attribute</title>
<link rel="preload" href="styles.css" as="style">
</head>
<body>
<p>Hello, World!</p>
</body>
</html>Example with modulepreload:
If you are preloading a JavaScript module, the rel attribute should be set to "modulepreload":
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Correct Usage of Link with Modulepreload</title>
<link rel="modulepreload" href="script.js" as="script">
</head>
<body>
<p>Hello, World!</p>
</body>
</html>Empty href attributes like href or href="" are invalid in HTML and can cause unpredictable behavior.
According to the HTML specification, the href attribute in elements such as the a and link tags must contain a valid URL or fragment reference. An empty or omitted href results in the attribute being ignored or possibly causing compatibility problems, especially in older browsers. If you intend the link to do nothing, use href="#" and prevent the default action with JavaScript, or avoid making it a link if no action is necessary.
Valid HTML for a regular link:
<a href="https://example.com">Visit Example</a>Valid usage for a link that does nothing:
<a href="#" onclick="return false;">Do nothing</a>If the element should not be clickable, use a different tag such as span or button:
<span>Not a link</span>or
<button type="button">Not a link</button>Avoid writing:
<a href>Invalid link</a>or
<a href="">Invalid link</a>as these will produce the validation warning you saw.
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>