HTML Guide

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.

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.

<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>

Spaces in the poster attribute value are not valid in URLs and must be percent-encoded as %20.

The poster attribute on the video element specifies an image to show until the user plays or seeks the video. Attribute values that represent URLs (such as in src, href, or poster) must use valid URI syntax, meaning spaces are not allowed. Spaces must be replaced with %20, or you can use a path that avoids spaces entirely.

Example — Incorrect:

<video controls poster="/img/video images/snapshot.png">
  <source src="/videos/sample.mp4" type="video/mp4">
</video>

Example — Fixed with percent-encoding:

<video controls poster="/img/video%20images/snapshot.png">
  <source src="/videos/sample.mp4" type="video/mp4">
</video>

Example — Fixed by removing spaces from the folder name:

<video controls poster="/img/video-images/snapshot.png">
  <source src="/videos/sample.mp4" type="video/mp4">
</video>

Always encode any space in URLs as %20 or avoid spaces in file and folder names.

<video> elements accept a width 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>