HTML Guide
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" />Learn more:
Last reviewed: October 27, 2022
Related W3C validator issues
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 attribute on the img element must be a positive integer representing the number of pixels.
The HTML img element’s width and height attributes are expected to specify image dimensions in pixels. According to the HTML Living Standard, these attributes accept only non-negative integers. These integers define the rendered dimension of the image, overriding the actual image size based on its native resolution. The value “auto” is not a valid integer, which leads to the validation error you’ve encountered.
Here is a correct usage example of the img element:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Valid Image Width</title>
</head>
<body>
<img src="example.jpg" alt="Example image" width="200" height="100">
</body>
</html>In the example above, the width is set to 200, and the height is set to 100. Both values are non-negative integers representing pixel dimensions. If you intend to maintain the image’s aspect ratio while adjusting another dimension, you can omit one of the attributes, and modern browsers will automatically adjust the aspect ratio based on the given dimension.
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:
-
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> -
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 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 HTML specification requires that the width and height attributes on <img> elements, when present, contain a string representing a non-negative integer — that is, a sequence of one or more ASCII digits like "0", "150", or "1920". An empty string ("") does not satisfy this requirement, so the W3C validator flags it as an error.
This issue commonly arises when:
- A CMS or templating engine outputs width="" or height="" because no dimension value was configured.
- JavaScript dynamically sets img.setAttribute("width", "") instead of removing the attribute.
- A developer adds the attributes as placeholders intending to fill them in later but forgets to do so.
Why it matters
Providing valid width and height attributes is one of the most effective ways to prevent Cumulative Layout Shift (CLS). Browsers use these values to calculate the image’s aspect ratio and reserve the correct amount of space before the image loads. When the values are empty strings, the browser cannot determine the aspect ratio, so no space is reserved — leading to layout shifts as images load in, which hurts both user experience and Core Web Vitals scores.
Beyond performance, invalid attribute values can cause unpredictable rendering behavior across browsers. Some browsers may ignore the attribute, others may interpret the empty string as 0, collapsing the image to zero pixels in that dimension. Standards-compliant HTML also improves accessibility by ensuring assistive technologies can parse the document reliably.
Examples
❌ Invalid: empty string values
<img src="photo.jpg" alt="A sunset over the ocean" width="" height="">Both width and height are set to empty strings, which is not valid.
✅ Fixed: provide actual dimensions
<img src="photo.jpg" alt="A sunset over the ocean" width="800" height="600">Replace the empty strings with the image’s actual pixel dimensions. These values should reflect the image’s intrinsic (natural) size. CSS can still be used to scale the image visually — the browser will use the width and height ratio to reserve the correct space.
✅ Fixed: remove the attributes entirely
<img src="photo.jpg" alt="A sunset over the ocean">If you don’t know the dimensions or prefer to handle sizing purely through CSS, remove the attributes altogether. An absent attribute is valid; an empty one is not.
❌ Invalid: only one attribute is empty
<img src="banner.jpg" alt="Promotional banner" width="1200" height="">Even if only one attribute has an empty value, the validation error will be triggered for that attribute.
✅ Fixed: both attributes with valid values
<img src="banner.jpg" alt="Promotional banner" width="1200" height="400">Fixing dynamic/template-generated markup
If a template language is outputting empty attributes, use a conditional to omit them when no value is available. For example, in a template:
<!-- Instead of always outputting the attributes: -->
<img src="photo.jpg" alt="Description" width="" height="">
<!-- Conditionally include them only when values exist: -->
<img src="photo.jpg" alt="Description" width="800" height="600">If you’re setting dimensions via JavaScript, remove the attribute rather than setting it to an empty string:
// ❌ Don't do this
img.setAttribute("width", "");
// ✅ Do this instead
img.removeAttribute("width");
// ✅ Or set a valid value
img.setAttribute("width", "800");A note on values
The width and height attributes only accept non-negative integers — whole numbers without units, decimals, or percentage signs. Values like "100px", "50%", or "3.5" are also invalid. Use plain integers like "100" or "600". If you need responsive sizing with percentages or other CSS units, apply those through CSS styles instead.
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.
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:
<img src="photo.jpg" alt="Dog" height="602.88" width="800.2">Corrected code without decimals:
<img src="photo.jpg" alt="Dog" height="603" width="800">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.
Attribute values for width and height on the object element must be a valid integer (pixels), not a percentage.
In HTML5, the width and height attributes for the <object> element must be specified in pixels as an integer value, such as height="500". Using percentage values like height="100%" is not allowed in the attribute according to the HTML living standard and will trigger a validation error. To set a percentage height, use the CSS width and height properties instead.
Correct usage (pixels via attribute):
<object data="example.pdf" type="application/pdf" width="600" height="400"></object>Correct usage (percentage via CSS):
<object data="example.pdf" type="application/pdf" style="width: 100%; height: 100%;"></object>Example of a full valid HTML document using CSS for 100% height:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Object Height Example</title>
<style>
html, body {
height: 100%;
margin: 0;
}
object {
height: 100%;
width: 100%;
display: block;
}
</style>
</head>
<body>
<object data="example.pdf" type="application/pdf"></object>
</body>
</html>The width and height attributes on <img> and <iframe> elements expect a digit to indicate the number of pixels. Ensure that this attribute contains only digits.
For example:
<!-- This is invalid because width is not a digit -->
<img width="225px" height="100px" alt="cat" src="cat.jpg" />
<!-- This is valid -->
<img width="225" height="100" alt="cat" src="cat.jpg" />The <table> element does not accept a height attribute. Use CSS instead.