HTML Validation

An ID must not contain whitespace

About This HTML Issue

The id attribute of an HTML element, used to identify the element when linking, scripting or styling, must be unique in the whole document and must not contain whitespace.

Technically, in HTML5, the value for an id attribute may contain any character, except whitespace characters. However, to avoid inadvertent errors, only ASCII letters, digits, _, and - should be used and the value for an id attribute should start with a letter.

Learn more:

→ MDN: the id global attribute

Last reviewed: September 02, 2024

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Related W3C Validator Issues (11)

Bad value “X” for attribute “for” on element “label”: An ID must not contain whitespace.

The <label> element represents a caption in a document, and it can be associated with a form input using the for attribute, which must be an ID. Document IDs cannot contain whitespace.

Example:

<form>
  <label for="user_name">Name</label>
  <input type="text" id="user_name" />
</form>

Bad value “” for attribute “for” on element “label”: An ID must not be the empty string.

The for attribute on a label element can’t be an empty string. This attribute is intended to specify which form element a label is associated with, and it must reference the ID of an existing form element. An empty string is neither a valid ID nor a meaningful association.

Explanation

Invalid HTML: <label for=""></label>

The for attribute expects the value to be the ID of a form element, such as an input, textarea, select, etc.

How to Fix

Identify the Form Element: Find the form element (input, textarea, select, etc.) that the label is supposed to be associated with.
Assign an ID to the Form Element: Ensure the form element has a unique ID.
Modify the Label’s for Attribute: Set the for attribute of the label to match the ID of the form element.

Example

Before Fix

<form>
    <label for="">Username:</label>
    <input type="text" name="username">
</form>

After Fix

<form>
    <label for="username">Username:</label>
    <input type="text" id="username" name="username">
</form>

Bad value “” for attribute “id” on element “select”: An ID must not be the empty string.

id
empty

Bad value “” for attribute “id” on element X: An ID must not be the empty string.

The validator reports “Bad value “” for attribute id on element X: An ID must not be the empty string” when any element includes an empty id attribute. Per the HTML standard, id is a global attribute used as a unique document-wide identifier. An empty identifier is not a valid value and is ignored by some features, leading to hard-to-debug issues.

This matters for accessibility and interoperability. Features that depend on IDs—fragment navigation (#target), <label for>, ARIA attributes like aria-labelledby/aria-controls, and DOM APIs such as document.getElementById()—require a non-empty, unique value. Empty IDs break these links, can degrade assistive technology output, and violate conformance, which may hide bugs across browsers.

How to fix:

If the element doesn’t need an identifier, remove the id attribute entirely.
If it needs one, provide a non-empty, unique value, e.g., id="main-content".
Ensure uniqueness across the page; each id must occur only once.
Use simple, predictable tokens: avoid spaces, prefer lowercase letters, digits, hyphens, and underscores (e.g., feature-1). While the spec allows a broad range of characters, sticking to URL- and selector-friendly characters avoids pitfalls.

Examples

Example that triggers the validator error (empty id)

<div id=""></div>

Correct: remove an unnecessary empty id

<div></div>

Correct: provide a meaningful, unique id

<section id="features"></section>

Problematic label association with empty id (invalid)

<label for="">Email</label>
<input type="email" id="">

Correct label–control association

<label for="email">Email</label>
<input type="email" id="email">

Correct ARIA relationship

<h2 id="pricing-heading">Pricing</h2>
<section aria-labelledby="pricing-heading">
  <p>Choose a plan.</p>
</section>

Correct fragment navigation target

<nav>
  <a href="#contact">Contact</a>
</nav>
<section id="contact">
  <h2>Contact us</h2>
</section>

Minimal full document (validated) demonstrating proper ids

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Valid IDs Example</title>
  </head>
  <body>
    <main id="main-content">
      <h1 id="page-title">Welcome</h1>
      <p>Jump to the <a href="#details">details</a>.</p>
      <section id="details">
        <h2>Details</h2>
      </section>
      <form>
        <label for="email">Email</label>
        <input id="email" type="email">
      </form>
    </main>
  </body>
</html>

Bad value X for attribute “id” on element “pattern”: Not a valid XML 1.0 name.

Duplicate ID “X”.

The first occurrence of ID “X” was here.

The “name” attribute is obsolete. Consider putting an “id” attribute on the nearest container instead.

In the days before HTML5, named anchors were used as a way to provide a link to a specific section of a document, for example:

<h2>
  <a name="section-5">Section 5</a>
</h2>

Now in HTML5, the name attribute is obsolete for <a> tags, and instead, you can use the id attribute of any element (not just <a>) as a way to navigate directly to it, for example:

<h2 id="section-5">Section 5</h2>

The “name” attribute on the “img” element is obsolete. Use the “id” attribute instead.

The warning regarding the use of the name attribute on the img element arises because the name attribute is considered obsolete in modern HTML. Historically, name was used to identify form controls and some other elements, but now it’s replaced by more standardized attributes like id.

To resolve this issue, replace the name attribute with the id attribute. The id attribute provides a unique identifier for the element within the document, which can be utilized for styling, scripting, or linking using fragment identifiers.

Here is an example of how to make this change:

HTML with obsolete name attribute:

<img src="example.jpg" name="myImage" alt="Descriptive text">

Updated HTML using the id attribute:

<img src="example.jpg" id="myImage" alt="Descriptive text">

In this modification:

The name="myImage" is replaced with id="myImage".
The remaining attributes like src and alt are retained for specifying the image source and providing alternative text, respectively.

The id attribute should be unique within the document, which ensures that JavaScript or CSS can target the element efficiently.

The “name” attribute on the “option” element is obsolete. Use the “id” attribute instead.

The <option> element no longer accepts a name attribute, which is now obsolete.

Example:

<select id="pet-select">
    <option value="">--Please choose an option--</option>
    <option value="dog">Dog</option>
    <option value="cat">Cat</option>
    <option value="hamster">Hamster</option>
</select>

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