HTML Guide
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.
Learn more:
Related W3C validator issues
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>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>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.
For rel="preload" and as="image" only, the imagesizes attribute is a sizes attribute that indicates to preload the appropriate resource used by an img element with corresponding values for its srcset and sizes attributes.
For rel="preload" and as="image" only, the imagesrcset attribute is a sourceset attribute that indicates to preload the appropriate resource used by an img element with corresponding values for its srcset and sizes attributes.
To query for the size of the viewport (or the page box on page media), the width, height and aspect-ratio media features should be used, rather than device-width, device-height and device-aspect-ratio, which refer to the physical size of the device regardless of how much space is available for the document being laid out. The device-* media features are also sometimes used as a proxy to detect mobile devices. Instead, authors should use media features that better represent the aspect of the device that they are attempting to style against.
The width media feature describes the width of the targeted display area of the output device. For continuous media, this is the width of the viewport including the size of a rendered scroll bar (if any).
In the following example, this media query expresses that the style sheet is only linked if the width of the viewport 768px maximum:
<link rel="stylesheet" media="only screen and (max-width: 768px)" href="styles.css">To query for the size of the viewport (or the page box on page media), the width, height and aspect-ratio media features should be used, rather than device-width, device-height and device-aspect-ratio, which refer to the physical size of the device regardless of how much space is available for the document being laid out. The device-* media features are also sometimes used as a proxy to detect mobile devices. Instead, authors should use media features that better represent the aspect of the device that they are attempting to style against.
The width media feature describes the width of the targeted display area of the output device. For continuous media, this is the width of the viewport including the size of a rendered scroll bar (if any).
In the following example, this media query expresses that the style sheet is only linked if the width of the viewport is greater than 768px:
<link rel="stylesheet" media="only screen and (min-width: 768px)" href="styles.css">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.
The media attribute on a <link> element specifies the media for which the linked resource is intended, using a valid media query. The value indicated is not a valid media query, which leads to a parse error.
Explanation
The media attribute is used to define what type of device the linked stylesheet should be applied to. Examples include common media types like screen, print, or all, as well as more complex media queries, such as (max-width: 600px) for responsive designs.
Correct Usage
-
Media Type Only:
You can specify general media types such as screen, print, or all.
<link rel="stylesheet" href="styles.css" media="screen"> -
Media Query:
For more precise control, use a media query to target specific conditions.
<link rel="stylesheet" href="styles.css" media="(max-width: 600px)">