HTML Guide

Bad value “X” for attribute “rel” on element “Y”: The string “X” is not an absolute URL.

The rel attribute defines the relationship between a linked resource and the current document. Valid on <link>, <a>, <area>, and <form>, the supported values depend on the element on which the attribute is found.

Here’s an example of using the rel attribute to link a document to a CSS stylesheet:

<link rel="stylesheet" href="default.css" />

Here’s an example os using the rel attribute to tell search engine spiders to ignore the link relationship with another document:

<a href="https://example.com" rel="nofollow">more info</a>

Learn more:

MDN - rel

Related W3C validator issues

A “link” element with an “as” attribute must have a “rel” attribute that contains the value “preload” or the value “modulepreload”.

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>

Attribute “maxlength” is only allowed when the input type is “email”, “password”, “search”, “tel”, “text”, or “url”.

Attribute “minlength” is only allowed when the input type is “email”, “password”, “search”, “tel”, “text”, or “url”.

The minlength attribute can be used on an input element to define a client-side validation for the maximum length allowed on an input without resorting to JavaScript.

This attribute is only allowed on elements of type email, password, search, tel, text, or url.

The minlength attribute defines the minimum number of characters (as UTF-16 code units) the user can enter into an <input> or <textarea>. This must be an integer value 0 or higher. If no minlength is specified, or an invalid value is specified, the input has no minimum length. This value must be less than or equal to the value of maxlength, otherwise the value will never be valid, as it is impossible to meet both criteria.

Here’s an example:

<label for="name">Enter your name (max 25 characters)</label>
<input type="text" minlength="25" id="name">

Attribute “pattern” is only allowed when the input type is “email”, “password”, “search”, “tel”, “text”, or “url”.

Attribute “required” is only allowed when the input type is “checkbox”, “date”, “datetime-local”, “email”, “file”, “month”, “number”, “password”, “radio”, “search”, “tel”, “text”, “time”, “url”, or “week”.

Bad value (true|false) for attribute “itemscope” on element “html”.

The itemscope attribute is a boolean attribute in HTML5, which means it does not take any values. Adding any value (such as true or false) will cause an error. When using boolean attributes, they should either be present or absent. If an attribute like itemscope is present, it is considered true.

Here’s how to correct the error:

Incorrect Usage:

<div itemscope="true">

Correct Usage:

<div itemscope>

Explanation:

Simply including the itemscope attribute without any value is the correct way to use it.
If you don’t want to use the itemscope attribute, just remove it from the tag.

Bad value “contact” for attribute “autocomplete” on element “input”: The string “contact” is not a valid autofill field name.

Bad value “dob” for attribute “type” on element “input”.

The type dob is not valid for an input. If you want to build a date picker field, you can use the native HTML input elements with type date, datetime-local, or a generic text input decorated with JavaScript and CSS.

In HTML, the type attribute for the <input> element specifies the type of input control that is to be displayed. The type attribute can have values like text, password, email, date, etc. Using an unsupported or invalid value like dob (which presumably stands for “date of birth”) will cause this validation error.

Here’s an example of how you can correct this issue by using a supported type attribute value for the date of birth input:

<label for="dob">Date of Birth:</label>
<input type="date" id="dob" name="dob">

In this corrected example, we’ve used the type="date" attribute value for the date of birth input. This is a valid type for handling dates in HTML forms. Replace the input type with a supported type according to the specific data you need to capture.

Alternatively you can use a JavaScript library to build a date picker on a generic text input. For example, the popular bootstrap-datepicker library will generate a date picker around a text input.

Bad value “false” for attribute “hidden” on element X.

All HTML elements may have the hidden boolean attribute set. When specified on an element, it indicates that the element is not yet, or is no longer, relevant, so browsers won’t render it.

Boolean attributes don’t accept values, its presence represents the true value and its absence represents the false value.

<!-- This is invalid because the hidden attribute should not have a value set -->
<div hidden="false"></div>

<!-- The correct way to hide a div is like this -->
<div hidden>This will be hidden</div>

<!-- And to show the element, we just don't hide it -->
<div>This won't be hidden</div>

Bad value for attribute “href” on element “a”: Illegal character in fragment: “#” is not allowed.

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>