HTML Guide

Bad value “” for attribute “maxlength” on element “textarea”: The empty string is not a valid non-negative integer.

Remove the empty maxlength and provide a valid non‑negative integer, or omit maxlength entirely.

The maxlength attribute limits the number of characters a user can enter. It must be a valid non-negative integer (0, 1, 2, …). An empty string, whitespace, or non-numeric value fails validation. When maxlength is omitted, the length is unlimited. When set to 0, no characters can be entered. The value applies to the textarea element itself, not its content.

Common fixes:

Set maxlength to a concrete number like 200.
If you don’t need a limit, remove the attribute.
Ensure templating doesn’t output an empty attribute (e.g., maxlength="").

HTML Examples

Invalid example (reproduces the validator error)

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Invalid maxlength</title>
</head>
<body>
  <form>
    <label for="msg">Message</label>
    <textarea id="msg" name="message" maxlength=""></textarea>
  </form>
</body>
</html>

Fixed example (valid integer or omit attribute)

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Valid maxlength</title>
</head>
<body>
  <form>
    <label for="msg">Message (max 200 chars)</label>
    <textarea id="msg" name="message" maxlength="200"></textarea>
  </form>
</body>
</html>

Learn more:

Last reviewed: August 19, 2025

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Related W3C validator issues

Bad value “virtual” for attribute “wrap” on element “textarea”.

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>

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

Attribute “type” not allowed on element “textarea” at this point.

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 “callto:X” for attribute “href” on element “a”: Illegal character in scheme data: space is not allowed.

Spaces are not permitted in the href value for phone links; the phone number must be a continuous string without spaces or slashes.

The href attribute of an anchor (<a>) element defines the link’s destination. For phone numbers, the proper URI scheme is tel:, not callto:. According to the HTML standard and the WHATWG Living Standard, the phone number should contain only digits and may use plus (+) or hyphen (-) characters for formatting, but it should not include spaces or slashes.

Incorrect HTML:

<a href="callto:07142/ 12 34 5">Call us</a>

Correct HTML:

<a href="tel:0714212345">Call us</a>

With country code and optional formatting:

<a href="tel:+49714212345">Call us</a>

For best compatibility and validation, always use the tel: scheme and ensure the phone number string contains only allowed characters.

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

Bad value “dialog” for attribute “role” on element “dialog”.

The dialog element does not require or permit a role="dialog" attribute according to HTML standards.

The <dialog> element has an implicit ARIA role of dialog, so adding role="dialog" is redundant and not valid per the specification. Instead, simply use the <dialog> element without an explicit role attribute.

Details:

According to the WHATWG HTML standard and ARIA specification, native <dialog> elements automatically have the correct role. Adding role="dialog" can cause HTML validation errors, as the validator interprets this as a misuse or redundancy.

Correct usage:

<dialog>
  <p>This is a dialog box.</p>
  <button>Close</button>
</dialog>

Incorrect usage (causes validation error):

<dialog role="dialog">
  <p>This is a dialog box.</p>
  <button>Close</button>
</dialog>

Removing the role="dialog" attribute resolves the W3C validation issue while maintaining accessibility.

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 “aria-controls” on element “a”: An IDREFS value must contain at least one non-whitespace character.

The validator error occurs when an element such as an a, button, or custom widget includes aria-controls="" (empty) or whitespace-only. The aria-controls attribute takes one or more space-separated id values (IDREFS). Each referenced id must exist exactly once in the same document. Leaving it empty violates the ARIA and HTML requirements and provides no usable relationship for assistive technologies.

Why this matters:

Accessibility: Screen readers rely on aria-controls to announce relationships between controls and controlled regions (e.g., a toggle and its panel). An empty value misleads AT or adds noise.
Standards compliance: HTML and ARIA require at least one non-whitespace id. Empty values cause validation failures.
Robustness: Incorrect references can confuse scripts and future maintainers, and break behavior when IDs change.

How to fix it:

Only add aria-controls when the element truly controls another region (show/hide, sort, update).
Ensure the controlled element has a unique id.
Set aria-controls to that id (or multiple space-separated IDs).
Keep the reference in sync if IDs change.
If nothing is controlled, remove aria-controls entirely.

Examples

Invalid: empty aria-controls (triggers the error)

<a href="#" aria-controls="">Toggle details</a>

Valid: control a single region

<div id="details-panel" hidden>
  Some details...
</div>
<a href="#details-panel" aria-controls="details-panel">Toggle details</a>

Valid: control multiple regions (space-separated IDs)

<section id="filters" hidden>...</section>
<section id="results" hidden>...</section>
<button type="button" aria-controls="filters results">Show filters and results</button>

Valid: remove when not needed

<a href="#">Toggle details</a>

Minimal complete document with proper usage

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>aria-controls Example</title>
  </head>
  <body>
    <button type="button" aria-controls="info" aria-expanded="false">Toggle info</button>
    <div id="info" hidden>
      Extra information.
    </div>
    <script>
      const btn = document.querySelector('button');
      const panel = document.getElementById(btn.getAttribute('aria-controls'));
      btn.addEventListener('click', () => {
        const expanded = btn.getAttribute('aria-expanded') === 'true';
        btn.setAttribute('aria-expanded', String(!expanded));
        panel.hidden = expanded;
      });
    </script>
  </body>
</html>

Tips:

Use aria-controls for functional relationships (control affects content), not just visual proximity.
Combine with aria-expanded when toggling visibility to convey state.
Verify that every id in aria-controls exists and is unique; avoid dynamic mismatches created by templating or component reuse.