HTML Guide for bad value

Empty type attributes are invalid on the <a> element; you must either remove the type attribute or provide a valid MIME type value.

The type attribute on an <a> (anchor) element specifies the MIME type of the linked resource. According to the WHATWG HTML specification and W3C validator, the value of the type attribute cannot be an empty string. It should be a valid MIME type, such as application/pdf or text/html. If you do not know the MIME type or it’s not relevant, you should omit the attribute.

Incorrect HTML example (causes validation error):

<a href="document.pdf" type="">Download PDF</a>

Correct HTML example (remove the invalid attribute):

<a href="document.pdf">Download PDF</a>

Correct HTML example (provide a valid MIME type):

<a href="document.pdf" type="application/pdf">Download PDF</a>

If the type is unknown or unnecessary, leaving it out is preferred to using an empty value.

The icon value for attribute role is not a valid ARIA role.

ARIA Roles: Overview

ARIA roles are used to enhance accessibility by clearly defining the role and purpose of an element for assistive technologies. However, there are defined roles that you need to adhere to:

• Standard roles include button, checkbox, alert, dialog, img, etc.
• There is no ARIA role named icon.

Solution

To fix the issue, you need to use a valid ARIA role that accurately describes the purpose of the span element. If your intention is to convey that the span contains an icon, you might want to reconsider whether you need a role at all. Often, purely decorative elements should not have a role, or you might use an img role if it conveys a meaningful image.

Here’s how you can address this:

1. No ARIA role (if purely decorative): If the icon is purely decorative and does not add meaningful content to your page, you should remove the role attribute entirely.

   <span class="icon"></span>

2. Using img role (if it represents an image): If the span represents an image or an icon that conveys meaningful information, you can use role="img" and provide a proper aria-label.

   <span class="icon" role="img" aria-label="Icon Description"></span>

3. Using an appropriate role (if interactive): If the icon is part of an interactive element, you might need a different role. For instance, if the icon is inside a button:

   <button>
      <span class="icon" aria-hidden="true"></span>
      Button text
   </button>

   Here, aria-hidden="true" is used to hide the decorative icon from screen readers as the text provides the necessary context.

The role attribute value navigation is invalid for a ul element, as it should be used with a nav element or similar suitable elements.

In HTML, the role attribute defines what an element represents in the context of accessible web technologies, primarily for assistive tools like screen readers. The nav element represents a section of a page intended for navigational links, and it inherently provides the role of navigation. If you want to make a ul element serve as navigation, it is more appropriate to use it inside a nav element, or alternatively, set a valid ARIA role on the element itself.

Detailed Explanation

HTML5 introduced a specific set of elements with implicit ARIA roles and behaviors, like the nav element, which implicitly has the navigation role. For backward compatibility or advanced use cases, developers might explicitly set ARIA roles using the role attribute. However, setting an invalid role can lead to accessibility issues, as seen with trying to assign navigation to a ul element.

Instead of applying the navigation role to a ul directly, wrap your ul with a nav element.

The allowed ARIA roles for an ul element are directory, group, listbox, menu, menubar, none, presentation, radiogroup, tablist, toolbar and tree, but not navigation.

Examples

Here is how you can use the nav element with a ul.

<nav>
  <ul>
    <li><a href="#home">Home</a></li>
    <li><a href="#about">About</a></li>
    <li><a href="#contact">Contact</a></li>
  </ul>
</nav>

Use role="none" (or remove the role) instead of role="presentation" on an img.

The role attribute maps elements to ARIA roles. For images, the valid way to make an image decorative is to either omit the ARIA role and use an empty alt attribute (alt=""), or use the ARIA role none. In ARIA 1.1, role="none" and role="presentation" are equivalent, but the W3C HTML Validator flags role="presentation" on img because the correct, accessible pattern is an empty alt to hide the image from assistive tech. Use alt="" alone, or pair it with role="none" if you need an explicit ARIA role. If the image conveys meaning, provide a descriptive alt and no role.

HTML Examples

Example that reproduces the issue

<img src="avatar.png" alt="" role="presentation">

Fixed examples

Decorative image (preferred minimal fix):

<img src="avatar.png" alt="">

Informative image:

<img src="chart.png" alt="Quarterly sales trend line chart">

sidebar is not a valid value for the role attribute according to the ARIA (Accessible Rich Internet Applications) specification.

The role attribute is used to define the purpose of an element for assistive technologies. Only certain predefined values are valid, such as navigation, complementary, main, banner, contentinfo, and others. There is no sidebar role in the ARIA or HTML specification. If you wish to indicate a sidebar, you should use the complementary role, which is intended for content that is tangentially related to the main content, such as a sidebar.

Incorrect Example:

<div role="sidebar">
<!-- Sidebar content -->

</div>

Correct Example:

<div role="complementary">
<!-- Sidebar content -->

</div>

Alternatively, you can use the aside element, which conveys the same meaning and does not require an explicit role attribute:

<aside>
<!-- Sidebar content -->

</aside>

Using either the role="complementary" on a generic container or the semantic <aside> element ensures your HTML is valid and accessible.

sidebar is not a valid value for the role attribute according to the ARIA (Accessible Rich Internet Applications) specification.

The role attribute is used to define the purpose of an element for assistive technologies. Only certain predefined values are valid, such as navigation, complementary, main, banner, contentinfo, and others. There is no sidebar role in the ARIA or HTML specification. If you wish to indicate a sidebar, you should use the complementary role, which is intended for content that is tangentially related to the main content, such as a sidebar.

Incorrect Example:

<div role="sidebar">
<!-- Sidebar content -->

</div>

Correct Example:

<div role="complementary">
<!-- Sidebar content -->

</div>

Alternatively, you can use the aside element, which conveys the same meaning and does not require an explicit role attribute:

<aside>
<!-- Sidebar content -->

</aside>

Using either the role="complementary" on a generic container or the semantic <aside> element ensures your HTML is valid and accessible.

The role attribute in HTML is used to define the accessibility role of an element, which helps assistive technologies understand the purpose or type of the element. The value tabpanel is not appropriate for a <ul> element, which is used for unordered lists.

The role of tabpanel is intended to be used with elements that represent a tab panel, which is part of a tabbed interface. A tabbed interface consists of elements with roles like tablist, tab, and tabpanel. Typically, tabpanel is used with containers that house the content associated with a tab, such as a <div>.

To fix this error, ensure that the tabpanel role is applied to the correct element. Here’s a simple example of how a tab interface can be structured correctly:

<div role="tablist" aria-label="Sample Tabs">
  <button role="tab" aria-controls="panel-1" id="tab-1">Tab 1</button>
  <button role="tab" aria-controls="panel-2" id="tab-2">Tab 2</button>
</div>

<div role="tabpanel" id="panel-1" aria-labelledby="tab-1" hidden>
  <p>Content for Tab 1.</p>
</div>

<div role="tabpanel" id="panel-2" aria-labelledby="tab-2" hidden>
  <p>Content for Tab 2.</p>
</div>

In this example:

• The role tablist is applied to the container element that directly contains the tab elements.
• Each button serving as a tab has the role of tab.
• Each tab panel, which contains the content for a tab, has the role of tabpanel.

Avoid using tabpanel on non-semantic or incorrectly associated elements like <ul>. Instead, use elements like <div> or <section> for tab panels, ensuring the roles align with the intended roles in a tabbed interface.

The attribute aria-hidden must have a value of "true" (without extra quotes) or "false", not "\"true\"" (with double quotes inside the value).

The aria-hidden attribute is used to indicate whether an element and its children should be accessible to assistive technologies (like screen readers). Valid values are the strings "true" or "false" (without embedded quotation marks). Using extra quotation marks causes the validator to flag a bad value because the attribute’s value is interpreted literally.

Incorrect Example:

<div aria-hidden='"true"'>This is hidden from assistive tech</div>

Correct Example:

<div aria-hidden="true">This is hidden from assistive tech</div>

or

<div aria-hidden="false">This is visible to assistive tech</div>

Remove any extra quotation marks around your attribute value to resolve the error.

Invalid value “virtual” was used for the wrap attribute on a textarea.

The wrap attribute controls how the browser wraps text when submitting a form. It accepts only two keyword values:

• soft (default): Lines are not hard-wrapped on submission; the server receives unwrapped lines.
• hard: The browser inserts CRLFs at wrap boundaries on submission; you must also set cols for hard to be valid.

virtual was a non-standard value used by some legacy browsers and is not allowed by the HTML Standard or the W3C validator. Remove wrap entirely to get soft wrapping, or set it to soft/hard as needed. If you choose hard, include a sensible cols value. You can also control visual wrapping with CSS like white-space if needed.

HTML Examples

Example: Reproduces the validator error

<!doctype html>
<html lang="en">
<head>
  <title>Textarea Wrap Error</title>
</head>
<body>
  <form>
    <label for="msg">Message</label>
    <textarea id="msg" name="msg" wrap="virtual"></textarea>
  </form>
</body>
</html>

Example: Valid fixes

<!doctype html>
<html lang="en">
<head>
  <title>Textarea Wrap Fix</title>
</head>
<body>
  <form>
    <label for="msg-soft">Message (soft)</label>
    <textarea id="msg-soft" name="msg" wrap="soft"></textarea>

    <label for="msg-hard">Message (hard)</label>
    <textarea id="msg-hard" name="msg" wrap="hard" cols="40" rows="5"></textarea>
  </form>
</body>
</html>

The async attribute is boolean: the presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value. As a boolean attribute, it does not need to be passed any value such as true or 1 to activate the async property.

For classic scripts, if the async attribute is present, then the classic script will be fetched in parallel to parsing and evaluated as soon as it is available.

For module scripts, if the async attribute is present then the scripts and all their dependencies will be executed in the defer queue, therefore they will get fetched in parallel to parsing and evaluated as soon as they are available.

<script async src="app.js"></script>

<script async type="module">
  /* JavaScript module code here */
</script>

Remove the unit; height on embed expects a non-negative integer (pixels) or a valid CSS length only when set via CSS, not the HTML attribute.

Detailed explanation

The embed element supports the presentational attributes width and height as unsigned integers representing CSS pixels. In HTML, the height attribute must be a number without a unit, for example 650. Supplying 650px violates the attribute’s value syntax and triggers the validator error.

If you want to use units or other CSS lengths (e.g., px, em, %, vh), set them with CSS via the style attribute or a stylesheet using the height property, not the HTML attribute.

HTML examples

Example reproducing the issue

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Embed height invalid</title>
</head>
<body>
  <embed src="file.pdf" type="application/pdf" width="800" height="650px">
</body>
</html>

Corrected example (HTML attribute as integer)

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Embed height fixed</title>
</head>
<body>
  <embed src="file.pdf" type="application/pdf" width="800" height="650">
</body>
</html>

Alternative corrected example (use CSS units)

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Embed height via CSS</title>
  <style>
    .viewer { width: 800px; height: 650px; }
  </style>
</head>
<body>
  <embed class="viewer" src="file.pdf" type="application/pdf">
</body>
</html>

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.

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>

The at symbol (@) should be percent-encoded as %40 in order to include it at an href attribute.

An href attribute with the value http:// is not a valid URL because the host section is missing.

The href attribute in the area element must contain a valid URL. A URL requires a scheme (http or https), followed by ://, and then a valid host (such as example.com). The value http:// lacks a host, making it invalid according to URL standards and resulting in a validation error. If you don’t want the area to navigate anywhere, you can use href="#" for a placeholder or omit the href attribute entirely. To link to an actual location, provide the complete URL, including the host.

Invalid example:

<map name="myMap">
  <area shape="rect" coords="30,23,183,191" href="http://" alt="" >
</map>

Valid example with a real host:

<map name="myMap">
  <area shape="rect" coords="30,23,183,191" href="http://example.com/" alt="" >
</map>

Valid example with a placeholder, no navigation:

<map name="myMap">
  <area shape="rect" coords="30,23,183,191" href="#" alt="" >
</map>

Replace http:// with a full URL or a suitable placeholder as needed for your application.

A <pattern> element has been found with an invalid ID. Check the format of the ID and ensure it does not start with a digit, full stop (.) or hyphen (-).

The <pattern> element is used within <svg> elements, which use XML 1.0 syntax. That syntax specifies that valid IDs only include designated characters (letters, digits, and a few punctuation marks), and do not start with a digit, a full stop (.) character, or a hyphen-minus (-) character.

The media attribute on a <link> element has not been recognized.

This attribute specified what media the linked resource is optimized for. As an example, the following will link a general stylesheet, and a specific one for printing:

<head>
  <link rel="stylesheet" type="text/css" href="general.css">
  <link rel="stylesheet" type="text/css" href="print.css" media="print">
</head>

Valid values for this attribute include:

• all. Default, used for all media.
• print. Used for printers and print previews.
• screen. Used for computer, tablets or smartphone screens.

Invalid value used for the multiple attribute on an input element.

The multiple attribute is a boolean attribute for certain input types (e.g., email, file). Boolean attributes must appear without a value (or with the same name as value in legacy cases), and they only work on specific types. Valid usage: <input type="email" multiple> or <input type="file" multiple>. Invalid usage includes multiple="1", multiple="true", or using multiple on unsupported types like text or number. The email type allows comma-separated addresses when multiple is present; the file type allows selecting more than one file.

HTML Examples

Example that reproduces the error

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Invalid multiple</title>
</head>
<body>
<!-- Invalid: value on boolean attribute, and wrong type -->

  <input type="text" name="tags" multiple="1">
</body>
</html>

Corrected example

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Valid multiple</title>
</head>
<body>
<!-- Valid: boolean attribute without a value on supported types -->

  <input type="email" name="recipients" multiple placeholder="name@example.com, other@example.com">

  <input type="file" name="attachments" multiple>
</body>
</html>

The value of the name attribute on an <iframe> should not start with an underscore (_).

Browsing context names that begin with an underscore are reserved keywords in HTML, like _blank, _self, _parent, and _top. Using these reserved names or any custom name starting with an underscore for the name attribute of an <iframe> can lead to unexpected behavior and is considered invalid HTML.

Here’s how to fix the issue:

Problematic Code

<iframe src="https://example.com" name="_example"></iframe>

Solution

To resolve this issue, you should use a valid value for the name attribute that does not start with an underscore.

Corrected Code

<iframe src="https://example.com" name="example"></iframe>

Steps:

1. Identify the iframe element with the invalid name attribute value that starts with an underscore.
2. Replace the name value with a valid identifier that does not start with _. Use letters, numbers, hyphens (-), and underscores (_) (but not at the beginning).

The novalidate attribute is boolean: the presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value. As a boolean attribute, it does not need to be passed any value such as true or 1 to activate the novalidate property.

This boolean attribute indicates that the form shouldn’t be validated when submitted. If this attribute is not set (and therefore the form is validated), it can be overridden by a formnovalidate attribute on a <button>, <input type="submit">, or <input type="image"> element belonging to the form.

Example:

<form method="post" novalidate>
  <label>User Name:
    <input name="user-name" autocomplete="name">
  </label>
  <button>Save</button>
</form>

The W3C Validator issue you’re seeing is because the ping attribute on the <a> (anchor) element is supposed to contain a space-separated list of URLs to which, when the hyperlink is activated, the browser will send ping requests. The ping attribute must only contain proper “http” or “https” URLs, that is, relative URLs are not allowed.

Example of Incorrect Code

<a href="https://example.com" ping="/track">Visit Example</a>

Examples of Correct Code

1. Proper Ping URL: Ensure the ping URL starts with http or https and is correctly formatted.

   <a href="https://example.com" ping="https://example.com/track">Visit Example</a>

2. Multiple URLs: If you have multiple URLs, they should be space-separated and each should be a valid URL.

   <a href="https://example.com" ping="https://example.com/track https://example.com/another-track">Visit Example</a>

Common Mistakes to Avoid

• Using invalid URL schemes like mailto: or /relative-path.
• Including broken URLs or URLs with protocols other than http or https.
• Incorrectly formatting multiple URLs; they must be space-separated, not comma-separated or otherwise.

Note

The ping property is not effective in Firefox and its usage may be limited due to privacy and security concerns.

Spaces in the poster attribute value are not valid in URLs and must be percent-encoded as %20.

The poster attribute on the video element specifies an image to show until the user plays or seeks the video. Attribute values that represent URLs (such as in src, href, or poster) must use valid URI syntax, meaning spaces are not allowed. Spaces must be replaced with %20, or you can use a path that avoids spaces entirely.

Example — Incorrect:

<video controls poster="/img/video images/snapshot.png">
  <source src="/videos/sample.mp4" type="video/mp4">
</video>

Example — Fixed with percent-encoding:

<video controls poster="/img/video%20images/snapshot.png">
  <source src="/videos/sample.mp4" type="video/mp4">
</video>

Example — Fixed by removing spaces from the folder name:

<video controls poster="/img/video-images/snapshot.png">
  <source src="/videos/sample.mp4" type="video/mp4">
</video>

Always encode any space in URLs as %20 or avoid spaces in file and folder names.

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>