Bad value “X” for attribute “novalidate” on element “form”.

There is no attribute validate on the <form> element. Perhaps you meant novalidate?

If the novalidate attribute is present on a <form>, indicates that the form is not to be validated during submission.

For example, while this form has a required attribute on its input, it won’t be enforced because form validation has been disabled using novalidate:

<form novalidate>
  <label>City: <input required name="city"></label>
  <input type="submit" />
</form>

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.

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.

The value contact is not a valid option for the autocomplete attribute on an <input> element.

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.

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.

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>

The action attribute on a <form> element is not a required attribute, but if specified, must be a valid, non-empty URL. For example:

<form action="register.php">
</form>

The issue here is related to the encoding of URLs. In HTML and URLs, special characters that have specific meanings need to be encoded to ensure that the URL is interpreted correctly. This process converts characters to their percent-encoded form, where a character is replaced by % followed by two hexadecimal digits representing the ASCII code of the character.

Explanation:

• The action attribute of a <form> element specifies where to send the form-data when a form is submitted.
• If the URL contains special characters (e.g., spaces, <, >, #, %, etc.), they need to be percent-encoded. For instance, a space character is encoded as %20.
• In this case, the validator is complaining about a % sign that is not correctly followed by two hexadecimal digits, which typically happens if the URL was not properly encoded.

How to Fix:

1. Check the URL: Look for any raw special characters that need encoding.
2. Correctly Encode the URL: Use online tools or libraries that provide URL encoding support to ensure that the URL is correctly percent-encoded.

Example:

Suppose you have the following incorrect form tag:

<form action="submit%data.html">
<!-- form elements here -->

</form>

The % in submit%data.html should be followed by two hexadecimal digits. If %data was intended to be a part of the URL, it should be encoded properly. Here is how to correct it:

<form action="submit%25data.html">
<!-- form elements here -->

</form>

If % should represent data, replace % with %25, which is the percent-encoded form of %. Always verify each special character is correctly encoded. Using this approach ensures that the URL in the action attribute is valid according to HTML standards.

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.