HTML Validation

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.)

About This HTML Issue

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">

Learn more:

Last reviewed: August 11, 2025

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1 min read

Category

HTML Validation

Engine

W3C Validator

Last reviewed

August 11, 2025

Resources

Related W3C Validator Issues (102)

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 for an img element requires valid CSS syntax, and auto is not an acceptable value within that attribute.

The sizes attribute allows you to specify a list of media conditions and corresponding sizes for the images. Each condition determines which size of the image should be displayed at different viewport widths, ensuring responsive image delivery. The syntax for sizes should be a comma-separated list of media queries followed by a value denoting the corresponding width of the image. This width value may be in pixels (px) or as a percentage (vw, vh), but auto is not valid in this context.

Here is a breakdown of a correct sizes attribute usage:

50vw: This denotes that the image should take up 50% of the viewport’s width.
(max-width: 600px) 100vw, 50vw: When the viewport is at most 600 pixels wide, the image should occupy the full width (100vw). Otherwise, it should take 50 percent of the viewport width.

Remove “auto” from your sizes value and provide a valid, contextually correct CSS value.

Example

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Responsive Images Example</title>
</head>
<body>
  <img
    src="image.jpg"
    sizes="(max-width: 472px) 100vw, 472px"
    srcset="image-small.jpg 300w, image-medium.jpg 600w, image-large.jpg 1000w"
    alt="A description of the image"
  >
</body>
</html>

In this example, if the viewport width is less than or equal to 472 pixels, the image will take up the entire width (100vw). For larger widths, the srcset specifies different image files for varying resolutions.

Bad value X for attribute “sizes” on element “img”: Expected units (one of “em”, “ex”, “ch”, “rem”, “cap”, “ic”, “vw”, “svw”, “lvw”, “dvw”, “vh”, “svh”, “lvh”, “dvh”, “vi”, “svi”, “lvi”, “dvi”, “vb”, “svb”, “lvb”, “dvb”, “vmin”, “svmin”, “lvmin”, “dvmin”, “vmax”, “svmax”, “lvmax”, “dvmax”, “cm”, “mm”, “q”, “in”, “pc”, “pt”, “px”) but found “w” at Y

The sizes attribute on the img element must use CSS length units (such as px, vw, em) and not image width descriptors like w.

The sizes attribute defines the intended display size of the image in CSS length units, which helps the browser select the appropriate image from the srcset. Use units like px (pixels) or vw (viewport width percentage), not w, which is used in the srcset descriptors. The srcset attribute specifies different image resources and their width descriptors (e.g., 860w), while sizes reflects the image’s display size in the layout.

Correct usage:

sizes="(min-width: 568px) 140px" tells the browser the image will be displayed as 140 pixels wide when the viewport is at least 568 pixels wide.
srcset="photo.png?w=860&q=90 860w" uses w as the descriptor for the image resource’s width.

HTML example:

<img
  alt=""
  sizes="(min-width: 568px) 140px"
  srcset="photo.png?w=860&amp;q=90 860w"
  src="photo.png?w=860&amp;q=90">

Summary:

Use CSS units like px, vw, etc. in the sizes attribute.
Use the w descriptor only in srcset, not in sizes.

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.

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>

<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 “sizes” on element “img”: Bad CSS number token: Expected a minus sign or a digit but saw “a” instead at “auto”.

The sizes attribute specifies the size of the image when it is displayed on different devices.

The error message is saying that the value auto is not a valid value for the sizes attribute.

To fix this issue, you need to replace the value auto with a valid size. You can use a width descriptor or a media query to specify the size for different device widths.

Here’s an example of using a width descriptor:

<img src="example.jpg" sizes="(max-width: 600px) 100vw, 50vw" />

This example sets the size of the image to 100% of the viewport width when the device width is less than or equal to 600px, and 50% of the viewport width for larger device widths.

Alternatively, you can remove the sizes attribute altogether and let the browser decide the best size for the image based on the viewport size.

<img src="example.jpg" />

If you do this, the browser will use the default sizes value of 100vw and will scale the image accordingly.

Bad value “” for attribute “sizes” on element “img”: Must not be empty.

sizes
img

An empty sizes attribute on an img element is invalid; the attribute must contain a valid value or be omitted.

The sizes attribute specifies the slot width that the browser should use for selecting the appropriate image from those available in srcset. It should only be used when the srcset attribute is present. An empty string is not a valid value—if you do not have any sizes to specify, the attribute should be removed entirely.

Correct usage:

Remove the empty sizes and srcset attributes if not needed.
If specifying, provide valid values such as "100vw" or "(max-width: 600px) 100vw, 50vw".

Incorrect example:

<img src="photo.jpg" srcset="photo-small.jpg 480w, photo-large.jpg 1200w" sizes="" alt="">

Corrected example (with a valid sizes value):

<img src="photo.jpg" srcset="photo-small.jpg 480w, photo-large.jpg 1200w" sizes="100vw" alt="">

Bad value for attribute “src” on element “img”: Illegal character in scheme data: “<” is not allowed.

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 “presentation” for attribute “role” on element “img”

Use role="none" (or remove the role) instead of role="presentation" on an img.

The role attribute maps elements to ARIA roles. For images, the valid way to make an image decorative is to either omit the ARIA role and use an empty alt attribute (alt=""), or use the ARIA role none. In ARIA 1.1, role="none" and role="presentation" are equivalent, but the W3C HTML Validator flags role="presentation" on img because the correct, accessible pattern is an empty alt to hide the image from assistive tech. Use alt="" alone, or pair it with role="none" if you need an explicit ARIA role. If the image conveys meaning, provide a descriptive alt and no role.

HTML Examples

Example that reproduces the issue

<img src="avatar.png" alt="" role="presentation">

Fixed examples

Decorative image (preferred minimal fix):

<img src="avatar.png" alt="">

Informative image:

<img src="chart.png" alt="Quarterly sales trend line chart">

Bad value X for attribute “sizes” on element “img”: Bad media condition: Parse Error at Y.

sizes contains an invalid media condition; the value must be a comma-separated list of media conditions with corresponding slot sizes, ending with an optional fallback length.

Detailed explanation:

The sizes attribute on img pairs a media condition with a slot size that represents the layout width of the image for that condition. Syntax: (<media-condition>) <length>[, ...], <length> where the last item can be a bare length fallback.
A media condition uses the same grammar as CSS media queries. It must be valid CSS, e.g., (min-width: 600px) or (width <= 50rem) and (orientation: landscape). Each condition must be enclosed in parentheses unless using logical operators combining proper conditions.

Common parse errors:

Missing parentheses: use (min-width: 600px), not min-width: 600px.
Invalid units or tokens: use px, em, rem, vw, etc.; avoid % in media conditions.
Missing slot size after a condition: (min-width: 600px) must be followed by a length like 600px.
Using px only for slot size without units or using percentages: slot size must be a length like 300px, 50vw, not 300.
Trailing comma or extra commas.
Misusing comparison syntax: use modern range syntax like (600px <= width <= 1000px) or the traditional form (min-width: 600px) and (max-width: 1000px). Do not write (min-width <= 600px).
Slot sizes must be lengths: px, em, rem, vw, vh, vmin, vmax, ch. Percentages are not allowed in sizes slot sizes.
The srcset widths (w descriptors) must correspond to the intrinsic widths of the image candidates, e.g., 400w, 800w. The browser picks one based on sizes.

HTML examples:

Correct usage with media conditions and fallback:

<img
  src="image-800.jpg"
  srcset="image-400.jpg 400w, image-800.jpg 800w, image-1200.jpg 1200w"
  sizes="(min-width: 900px) 50vw, (min-width: 600px) 66vw, 100vw"
  alt="Decorative pattern">

Using range syntax and avoiding common mistakes:

<img
  src="hero-1600.jpg"
  srcset="hero-800.jpg 800w, hero-1200.jpg 1200w, hero-1600.jpg 1600w"
  sizes="(800px <= width < 1200px) 80vw, (width >= 1200px) 50vw, 100vw"
  alt="Hero banner">

Minimal fixed example for a typical error (missing parentheses and slot size): Incorrect:

<img src="pic-800.jpg" srcset="pic-400.jpg 400w, pic-800.jpg 800w" sizes="min-width: 600px, 100vw" alt="Sample">

Correct:

<img src="pic-800.jpg" srcset="pic-400.jpg 400w, pic-800.jpg 800w" sizes="(min-width: 600px) 50vw, 100vw" alt="Sample">

Example avoiding invalid tokens and commas:

<img
  src="avatar-256.png"
  srcset="avatar-128.png 128w, avatar-256.png 256w"
  sizes="(orientation: landscape) 30vw, 50vw"
  alt="User avatar">

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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.)

About This HTML Issue

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)

Find issues like this automatically

Related W3C Validator Issues (102)

Example

HTML examples

Example causing the error

Corrected example (encode brackets)

Corrected example (avoid brackets)

HTML Examples

Example that reproduces the issue

Fixed examples

Detailed explanation:

Common parse errors:

HTML examples:

Reproduce the issue (invalid: `sizes` + `x` descriptors)