Bad value X for attribute “srcset” on element “source”: Expected number greater than zero but found “0” at Y.

The srcset attribute lets browsers intelligently choose which image to load based on the viewport size and device pixel ratio. Each entry in a srcset consists of a URL followed by either a width descriptor (like 300w) or a pixel density descriptor (like 2x). When using width descriptors, the value represents the intrinsic pixel width of the image file — that is, the actual width of the image as stored on disk.

A width descriptor of 0w violates the HTML specification, which requires width descriptors to be integers greater than zero. A zero-width image cannot meaningfully participate in the browser’s source selection process. The browser uses these width values in combination with the sizes attribute to calculate which image best fits the current layout — a value of zero would break this calculation entirely.

This issue commonly occurs when:

• Image dimensions are dynamically generated and a fallback of 0 is used for missing data.
• A placeholder or empty state is accidentally included in the srcset.
• A CMS or build tool outputs a 0w descriptor for images whose dimensions weren’t computed.

Why it matters

• Standards compliance: The HTML specification explicitly requires width descriptors to be positive integers. Validators will flag 0w as an error.
• Browser behavior: While browsers may silently ignore the invalid entry, you can’t rely on consistent handling across all browsers and versions. The image selection algorithm may behave unpredictably.
• Performance: A well-formed srcset is key to responsive image loading. Invalid descriptors can prevent browsers from selecting the optimal image, leading to unnecessarily large downloads or poor image quality.

How to fix it

1. Open the image file associated with the 0w descriptor and check its actual pixel width using an image editor or the command line.
2. Replace 0w with the correct width (e.g., 150w for a 150-pixel-wide image).
3. If the image is truly zero-width or a placeholder, remove that entry from the srcset entirely.
4. Ensure every remaining entry has a unique, positive width descriptor.

Examples

❌ Invalid: width descriptor of 0w

<picture>
  <source
    srcset="/images/icon_placeholder.png 0w,
            /images/icon_large.png 600w"
    media="(max-width: 600px)">
  <img src="/images/icon_fallback.png" alt="App logo">
</picture>

The 0w descriptor triggers the validation error because zero is not a valid width.

✅ Fixed: all width descriptors are positive

<picture>
  <source
    srcset="/images/icon_small.png 300w,
            /images/icon_large.png 600w"
    media="(max-width: 600px)">
  <img src="/images/icon_fallback.png" alt="App logo">
</picture>

Each entry now has a meaningful width descriptor (300w and 600w) that reflects the actual pixel width of the corresponding image.

❌ Invalid: 0w on an <img> element

<img
  srcset="/images/hero_tiny.jpg 0w,
          /images/hero_medium.jpg 800w,
          /images/hero_large.jpg 1200w"
  sizes="100vw"
  src="/images/hero_medium.jpg"
  alt="Mountain landscape">

✅ Fixed: placeholder entry removed or corrected

If the tiny image is 400 pixels wide, use 400w:

<img
  srcset="/images/hero_tiny.jpg 400w,
          /images/hero_medium.jpg 800w,
          /images/hero_large.jpg 1200w"
  sizes="100vw"
  src="/images/hero_medium.jpg"
  alt="Mountain landscape">

Alternatively, if the image doesn’t belong in the set at all, simply remove it:

<img
  srcset="/images/hero_medium.jpg 800w,
          /images/hero_large.jpg 1200w"
  sizes="100vw"
  src="/images/hero_medium.jpg"
  alt="Mountain landscape">

When using a build tool or CMS that generates srcset values dynamically, add a check to filter out any entries where the computed width is zero or missing before rendering the attribute. This prevents the invalid markup from reaching production.

Learn more:

MDN Reference

MDN: srcset property

→

MDN Reference

MDN: HTML source element

→

MDN Reference

MDN: Responsive images - srcset attribute

→

WHATWG Spec

WHATWG: Embedded content

→