Bad value X for attribute “sizes” on element “img”: Bad CSS number token: Expected a minus sign or a digit but saw “a” instead at “auto,”.

The sizes attribute works together with srcset to help the browser choose the most appropriate image source for the current layout. It accepts a comma-separated list of entries, where each entry is an optional media condition followed by a CSS length value (called a “source size value”). The browser evaluates these entries to determine the intended display width of the image before it downloads it. Valid length values include viewport-relative units like 100vw, absolute units like 472px, or calc() expressions like calc(100vw - 2rem).

The value auto was not part of the original HTML specification for the sizes attribute. However, the sizes="auto" value has been added to the HTML living standard specifically for use with lazy-loaded images (loading="lazy"). When both loading="lazy" and sizes="auto" are present, the browser can defer size calculation until layout time, since the image won’t be fetched immediately anyway. Some validators may not yet recognize this newer addition, or the error may appear because auto is being used without loading="lazy", or combined incorrectly with other size entries like sizes="auto, 100vw".

This validation error matters for several reasons. First, if the browser doesn’t understand the sizes value, it may fall back to a default of 100vw, which could cause it to download a larger image than necessary, hurting performance. Second, malformed attribute values can lead to unpredictable behavior across different browsers. Third, standards compliance ensures your markup works reliably now and in the future.

How to Fix

You have a few options depending on your situation:

1. Replace auto with a valid CSS length. If you know the intended display size of the image, specify it directly. This is the most broadly compatible approach.

2. Use sizes="auto" only with loading="lazy". If you want the browser to automatically determine the size, ensure you also include loading="lazy" and a width attribute on the image. Note that some validators may still flag this until they update their rules.

3. Remove auto from a comma-separated list. If you have something like sizes="auto, (max-width: 600px) 100vw, 50vw", remove the auto entry entirely.

Examples

Incorrect: Using auto without lazy loading

This triggers the validation error because auto is not a valid CSS length in the traditional sizes syntax.

<img
  src="image.jpg"
  srcset="image-small.jpg 300w, image-medium.jpg 600w, image-large.jpg 1000w"
  sizes="auto, 100vw"
  alt="A scenic mountain landscape"
>

Fixed: Using a valid CSS length value

Replace auto with a concrete size or a set of media-conditioned sizes.

<img
  src="image.jpg"
  srcset="image-small.jpg 300w, image-medium.jpg 600w, image-large.jpg 1000w"
  sizes="(max-width: 472px) 100vw, 472px"
  alt="A scenic mountain landscape"
>

In this example, when the viewport is 472 pixels wide or smaller, the image takes up the full viewport width (100vw). For wider viewports, the browser knows the image will display at 472px wide and selects the best source from srcset accordingly.

Fixed: Using auto with lazy loading

If you want the browser to determine the display size automatically, pair sizes="auto" with loading="lazy" and explicit dimensions.

<img
  src="image.jpg"
  srcset="image-small.jpg 300w, image-medium.jpg 600w, image-large.jpg 1000w"
  sizes="auto"
  width="600"
  height="400"
  loading="lazy"
  alt="A scenic mountain landscape"
>

The width and height attributes help the browser reserve the correct space in the layout, and loading="lazy" allows the browser to defer both loading and size calculation until the image is near the viewport.

Fixed: Using calc() for dynamic sizing

If your image sits inside a container with padding, you can use calc() for a precise size hint.

<img
  src="image.jpg"
  srcset="image-small.jpg 300w, image-medium.jpg 600w, image-large.jpg 1000w"
  sizes="calc(100vw - 2rem)"
  alt="A scenic mountain landscape"
>

Learn more:

MDN Reference

MDN: HTMLImageElement sizes property

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MDN Reference

MDN: <img>: The Image Embed element

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web.dev Article

Web.dev: responsive images

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