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.
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Last reviewed: October 27, 2022
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The srcset attribute requires a width descriptor (w) or pixel density descriptor (x) for each image candidate when the sizes attribute is present.
When using the sizes attribute on an img element, each entry in srcset must include either a width descriptor (e.g., 860w) or a pixel density descriptor (e.g., 2x). This tells browsers how to select the most appropriate image source for the current viewport or display density. Omitting the descriptor leads to HTML validation errors and unexpected image selection.
Correct usage with width descriptors:
<img
alt=""
sizes="(min-width:568px) 140px"
srcset="photo.png?w=860&q=90 860w"
src="photo.png?w=860&q=90">Correct usage with pixel density descriptors (if sizes is removed):
<img
alt=""
srcset="photo.png?w=860&q=90 2x"
src="photo.png?w=860&q=90">Key points:
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.
A <source> element inside a <picture> that is followed by another <source> or an <img> with srcset must include a media and/or type attribute.
The <source> element is used inside <picture> for responsive images, allowing different resources to be loaded based on conditions such as viewport width (media) or image format (type). According to the HTML standard, when multiple <source> elements are present (or a following <img> with srcset), each <source> must provide a media and/or type attribute so the browser can choose the appropriate resource based on those hints.
Without media or type, the browser cannot distinguish when to use each source, which can lead to validation errors and unexpected rendering behavior.
Incorrect example (causes the validator error):
<picture>
<source srcset="image1.webp">
<source srcset="image2.jpg">
<img alt="" src="fallback.jpg" srcset="fallback2x.jpg 2x">
</picture>Correct examples (fixing the error):
<picture>
<source srcset="image1.webp" type="image/webp">
<source srcset="image2.jpg" type="image/jpeg">
<img alt="" src="fallback.jpg" srcset="fallback2x.jpg 2x">
</picture>or
<picture>
<source srcset="image-small.jpg" media="(max-width: 600px)">
<source srcset="image-large.jpg" media="(min-width: 601px)">
<img alt="" src="fallback.jpg" srcset="fallback2x.jpg 2x">
</picture>By specifying the media and/or type attributes for each <source>, you satisfy the HTML standard and resolve the W3C validator issue.
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 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.
Replace square brackets in srcset URLs or percent-encode them.
The img element’s srcset expects valid URLs for each image candidate. According to the URL Standard, unescaped square brackets are not allowed in the path or query of an HTTP(S) URL used in HTML attributes like srcset. They must be either removed, replaced, or percent-encoded.
<img
src="image.jpg"
srcset="image.jpg?size=[small] 1x, image@2x.jpg?size=[large] 2x"
alt="Sample"><img
src="image.jpg"
srcset="image.jpg?size=%5Bsmall%5D 1x, image@2x.jpg?size=%5Blarge%5D 2x"
alt="Sample"><img
src="image.jpg"
srcset="image.jpg?size=small 1x, image@2x.jpg?size=large 2x"
alt="Sample">srcset contains candidates without a width descriptor while sizes is present, so each candidate must use a width (w) descriptor.
When an img has sizes, every srcset candidate must include a width descriptor like 320w, not a pixel density descriptor like 1x. Mixed descriptors are not allowed in the same srcset. Use either:
The browser uses sizes to map CSS layout width to the best w candidate. Without sizes, density (x) can be used, but not together with sizes.
<img
src="photo-640.jpg"
srcset="photo-640.jpg 1x, photo-1280.jpg 2x"
sizes="(max-width: 600px) 100vw, 600px"
alt="Sample photo"><img
src="photo-640.jpg"
srcset="photo-320.jpg 320w, photo-640.jpg 640w, photo-1280.jpg 1280w"
sizes="(max-width: 600px) 100vw, 600px"
alt="Sample photo"><img
src="photo-640.jpg"
srcset="photo-640.jpg 1x, photo-1280.jpg 2x"
alt="Sample photo">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:
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.
<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.
<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.
<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.
<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.
<img src="banner.jpg" alt="Promotional banner" width="1200" height="400">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");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.
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