HTML Guide
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
idattribute entirely. -
If it needs one, provide a non-empty, unique value, e.g.,
id="main-content". -
Ensure uniqueness across the page; each
idmust 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>Learn more:
Last reviewed: February 13, 2026
Related W3C validator issues
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>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.
The id attribute is used to identify a single element within a document, it’s not required, but if used it must be unique, and must not be an empty string.
An empty string for the min attribute on an input element is invalid; it must be a valid number.
The min attribute specifies the minimum value an <input> element can accept when using types such as number, range, date, or datetime-local. According to the HTML specification, the value for min must be a valid floating point number (or a valid date/time string for date inputs). Setting min="" (an empty string) is invalid and will trigger validator errors.
HTML examples
Invalid usage:
<input type="number" min="" max="10">Valid usage (set min to a specific number, or omit it if no minimum is required):
<input type="number" min="0" max="10">or, if no minimum restriction is needed:
<input type="number" max="10">Always provide a valid number for min or remove the attribute entirely if a minimum is not needed.
The attributes width and height of <iframe> elements expect a non-negative integer, so an empty string is not allowed. Either define the correct dimension, or remove this attribute.
The HTML specification requires that the width and height attributes on <img> elements, when present, contain a string representing a non-negative integer — that is, a sequence of one or more ASCII digits like "0", "150", or "1920". An empty string ("") does not satisfy this requirement, so the W3C validator flags it as an error.
This issue commonly arises when:
- A CMS or templating engine outputs width="" or height="" because no dimension value was configured.
- JavaScript dynamically sets img.setAttribute("width", "") instead of removing the attribute.
- A developer adds the attributes as placeholders intending to fill them in later but forgets to do so.
Why it matters
Providing valid width and height attributes is one of the most effective ways to prevent Cumulative Layout Shift (CLS). Browsers use these values to calculate the image’s aspect ratio and reserve the correct amount of space before the image loads. When the values are empty strings, the browser cannot determine the aspect ratio, so no space is reserved — leading to layout shifts as images load in, which hurts both user experience and Core Web Vitals scores.
Beyond performance, invalid attribute values can cause unpredictable rendering behavior across browsers. Some browsers may ignore the attribute, others may interpret the empty string as 0, collapsing the image to zero pixels in that dimension. Standards-compliant HTML also improves accessibility by ensuring assistive technologies can parse the document reliably.
Examples
❌ Invalid: empty string values
<img src="photo.jpg" alt="A sunset over the ocean" width="" height="">Both width and height are set to empty strings, which is not valid.
✅ Fixed: provide actual dimensions
<img src="photo.jpg" alt="A sunset over the ocean" width="800" height="600">Replace the empty strings with the image’s actual pixel dimensions. These values should reflect the image’s intrinsic (natural) size. CSS can still be used to scale the image visually — the browser will use the width and height ratio to reserve the correct space.
✅ Fixed: remove the attributes entirely
<img src="photo.jpg" alt="A sunset over the ocean">If you don’t know the dimensions or prefer to handle sizing purely through CSS, remove the attributes altogether. An absent attribute is valid; an empty one is not.
❌ Invalid: only one attribute is empty
<img src="banner.jpg" alt="Promotional banner" width="1200" height="">Even if only one attribute has an empty value, the validation error will be triggered for that attribute.
✅ Fixed: both attributes with valid values
<img src="banner.jpg" alt="Promotional banner" width="1200" height="400">Fixing dynamic/template-generated markup
If a template language is outputting empty attributes, use a conditional to omit them when no value is available. For example, in a template:
<!-- Instead of always outputting the attributes: -->
<img src="photo.jpg" alt="Description" width="" height="">
<!-- Conditionally include them only when values exist: -->
<img src="photo.jpg" alt="Description" width="800" height="600">If you’re setting dimensions via JavaScript, remove the attribute rather than setting it to an empty string:
// ❌ Don't do this
img.setAttribute("width", "");
// ✅ Do this instead
img.removeAttribute("width");
// ✅ Or set a valid value
img.setAttribute("width", "800");A note on values
The width and height attributes only accept non-negative integers — whole numbers without units, decimals, or percentage signs. Values like "100px", "50%", or "3.5" are also invalid. Use plain integers like "100" or "600". If you need responsive sizing with percentages or other CSS units, apply those through CSS styles instead.
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>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 id attribute is used to identify a single element within a document, and is required to be unique. Check the document for repeated IDs.
Element IDs in an HTML document must be unique. The HTML validator is indicating the first occurrence of an ID that is repeated. Check the details for that issue to see web pages affected, and the elements within them, to fix that ID repetition.