HTML Guide

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 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 width and height attributes on <img> elements expect a digit to specify the dimension in pixels. It should not contain units, letters or percent signs.

You can achieve this using CSS instead, for example:

<!-- Invalid syntax, the height attribute expects only digits -->
<img src="photo.jpg" alt="cat" height="auto" />

<!-- Valid syntax using CSS -->
<img src="photo.jpg" alt="cat" style="height: auto"  />

The sizes attribute is used to complement the srcset attribute on an <img> tag for responsive images. When this attribute is present, all image candidates must specify its width.

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 attributes width and height of <img> elements expect a non-negative integer, so an empty string is not allowed. Either define the correct dimension, or remove this attribute.

<video> elements accept a height attribute to specify the width in CSS pixels. This value can only be an integer, it should not contain units or %. If you need to specify a percentage width, you can do that with CSS.

Here’s an example of setting width and height on a video element.

<video controls width="640" height="480">
  <source src="/media/cc0-videos/flower.webm" type="video/webm">
</video>

The sizes attribute is used to complement the srcset attribute on an <img> tag for responsive images. When this attribute is present, all image candidates must specify its width.

The attributes width and height on an iframe expect a valid positive integer without any decimals.

Here’s an example of incorrect code where decimals are being used for dimension attributes:

<iframe src="example.html" height="602.88" width="800.2"></iframe>

Corrected code without decimals:

<iframe src="example.html" height="603" width="800"></iframe>

In the corrected code, the width and height values has been changed to a whole number, which conforms to the standard integer value expected by the W3C validator.