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.)

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 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.

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">

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.

All values in the srcset attribute must include a width descriptor (such as 300w) when the sizes attribute is present.

The srcset attribute is used to provide multiple image sources for responsive images. Each image candidate string in srcset must specify the image’s width (e.g., 600w) or pixel density (e.g., 2x). When you use a sizes attribute, all srcset candidates must use width descriptors (w).

Example of incorrect usage:

<img
  src="/img/pic1.jpg"
  srcset="/img/pic1.jpg"
  sizes="(max-width: 600px) 100vw, 600px"
  alt=""
>

This example is invalid because the srcset value does not include a width descriptor.

Corrected usage with width descriptors:

<img
  src="/img/pic1.jpg"
  srcset="/img/pic1.jpg 600w"
  sizes="(max-width: 600px) 100vw, 600px"
  alt=""
>

If you have multiple image sizes, include each with its corresponding width:

<img
  src="/img/pic1.jpg"
  srcset="
    /img/pic1_small.jpg 300w,
    /img/pic1.jpg 600w
  "
  sizes="(max-width: 600px) 100vw, 600px"
  alt=""
>

Always match each image URL in srcset with a width (w) or pixel density (x) descriptor if appropriate for your layout.