HTML check: Bad value for attribute “srcset” on element “source”: No width specified for image X. (When the “sizes” attribute is present, all image candidate strings must specify a width.)

Bad value X for attribute “srcset” on element “img”: No width specified for image Y. (When the “sizes” attribute is present, all image candidate strings must specify a width.)

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&amp;q=90 860w"
  src="photo.png?w=860&amp;q=90">

Correct usage with pixel density descriptors (if sizes is removed):

<img
  alt=""
  srcset="photo.png?w=860&amp;q=90 2x"
  src="photo.png?w=860&amp;q=90">

Key points:

With sizes, use width descriptors (e.g., 860w).
Without sizes, you may use pixel density descriptors (e.g., 2x).
Always use either px or w units in the sizes attribute values; do not use w.

Bad value X for attribute “srcset” on element “source”: No width specified for image Y. (When the “sizes” attribute is present, all image candidate strings must specify a width.)

srcset
width

A “source” element that has a following sibling “source” element or “img” element with a “srcset” attribute must have a “media” attribute and/or “type” attribute.

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.

Bad value “auto” for attribute “height | weight” on element “img”: Expected a digit but saw “a” instead.

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"  />

Bad value “auto” for attribute “width” on element “img”: Expected a digit but saw “a” instead.

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.

Bad value “auto” for attribute “width” on element “video”: Expected a digit but saw “a” instead.

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.

Bad value for attribute “srcset” on element “img”: Bad image-candidate URL: Illegal character in query: “[” is not allowed.

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.

Use safe characters in query parameters (e.g., hyphens or underscores instead of brackets).
If brackets must remain for backend reasons, percent-encode them: [ -> %5B, ] -> %5D.
Ensure each image candidate follows the URL [whitespace] descriptor pattern (e.g., 2x, 300w) with commas separating candidates.

HTML examples

Example causing the error

<img
  src="image.jpg"
  srcset="image.jpg?size=[small] 1x, image@2x.jpg?size=[large] 2x"
  alt="Sample">

Corrected example (encode brackets)

<img
  src="image.jpg"
  srcset="image.jpg?size=%5Bsmall%5D 1x, image@2x.jpg?size=%5Blarge%5D 2x"
  alt="Sample">

Corrected example (avoid brackets)

<img
  src="image.jpg"
  srcset="image.jpg?size=small 1x, image@2x.jpg?size=large 2x"
  alt="Sample">

Bad value for attribute “srcset” on element “img”: Expected width descriptor but found X at Y. (When the “sizes” attribute is present, all image candidate strings must specify a width.)

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:

Width descriptors with sizes (e.g., 320w, 640w, 1024w)
Density descriptors without sizes (e.g., 1x, 2x)

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.

HTML examples

Reproduce the issue (invalid: sizes + x descriptors)

<img
  src="photo-640.jpg"
  srcset="photo-640.jpg 1x, photo-1280.jpg 2x"
  sizes="(max-width: 600px) 100vw, 600px"
  alt="Sample photo">

Fix using width descriptors with sizes (valid)

<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">

Alternative fix: remove sizes and use density descriptors (valid)

<img
  src="photo-640.jpg"
  srcset="photo-640.jpg 1x, photo-1280.jpg 2x"
  alt="Sample photo">

Bad value “” for attribute “(width|height)” on element “iframe”: The empty string is not a valid non-negative integer.

Bad value “” for attribute “(width|height)” on element “img”: The empty string is not a valid non-negative integer.

HTML Guide

Bad value for attribute “srcset” on element “source”: No width specified for image X. (When the “sizes” attribute is present, all image candidate strings must specify a width.)

Related W3C validator issues

HTML examples

Example causing the error

Corrected example (encode brackets)

Corrected example (avoid brackets)

HTML examples

Reproduce the issue (invalid: sizes + x descriptors)

Fix using width descriptors with sizes (valid)

Alternative fix: remove sizes and use density descriptors (valid)