HTML Guide

An empty value for the poster attribute on a video element is not valid; the attribute must contain a non-empty URL.

The poster attribute specifies an image to show before the user plays the video. According to the HTML living standard and W3C specifications, if the poster attribute is present, it must have a non-empty value that is a valid URL to an image resource. Using poster="" is invalid and triggers validator errors.

If you do not want to show any poster image, simply omit the poster attribute altogether. If you want to show an image, provide a valid image URL as the value.

Valid examples:

Video without a poster (omit the attribute):

<video controls>
  <source src="movie.mp4" type="video/mp4">
  Your browser does not support the video tag.
</video>

Video with a poster image:

<video controls poster="thumbnail.jpg">
  <source src="movie.mp4" type="video/mp4">
  Your browser does not support the video tag.
</video>

Invalid (causes error):

<video controls poster="">
  <source src="movie.mp4" type="video/mp4">
  Your browser does not support the video tag.
</video>

Always either use a valid URL in the poster attribute or omit it entirely to ensure HTML validity.

The controlslist attribute on the <video> element is experimental, and not yet valid in the current HTML5 standard, so you should use it with caution as it may not be supported by all browsers.

The height attribute on the <video> element must be a non-negative integer representing the height in CSS pixels. The value “auto” is not a valid value for this attribute. To resolve the issue, set the height attribute to a specific numeric value or adjust the height using CSS instead.

Example with a numeric height value:

<video width="640" height="360" controls>
  <source src="movie.mp4" type="video/mp4">
  Your browser does not support the video tag.
</video>

Alternatively, control the height using CSS:

<video width="640" controls style="height: auto;">
  <source src="movie.mp4" type="video/mp4">
  Your browser does not support the video tag.
</video>

In the CSS approach, “auto” can be used, but it should not be part of the HTML attributes.

The error message is indicating that the width attribute of the <video> element has an invalid value. According to the HTML specification, the width attribute expects a non-negative integer value, representing the pixel width of the video.

The value "auto" is not valid for the width attribute. Instead, specify a full number that indicates the pixel width of the video. If you want the video to be responsive without specifying a fixed width, you can use CSS to achieve that.

Here are two ways to resolve this:

1. Specify a valid pixel value for width:

   <video width="640" height="360" controls>
     <source src="video.mp4" type="video/mp4">
     Your browser does not support the video tag.
   </video>

2. Use CSS for a responsive video player:

   Instead of using the width attribute, use CSS to set the width of the video element. This allows the video to adjust its size according to the container or viewport.

   <style>
     .responsive-video {
       width: 100%;
       height: auto;
     }
   </style>
   
   <video class="responsive-video" controls>
     <source src="video.mp4" type="video/mp4">
     Your browser does not support the video tag.
   </video>

In the second example, the video will scale based on its containing element, maintaining its aspect ratio due to the height: auto; CSS rule. This approach offers more flexibility for responsive design.

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>