HTML Validation

Bad value X for attribute “sizes” on element “source”: 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 Y at Z.

sizes
source

About This HTML Issue

Use a valid media length in the sizes attribute; each slot’s length must include a unit (e.g., px, vw) and cannot be unitless.

Detailed explanation

The sizes attribute on responsive images (<img> or <source> within <picture>) specifies, for each media condition, the slot size that the image will occupy in the layout. Each size value must be a valid CSS length with units, or the keyword auto (for <img> only; not useful with <source>). Unitless numbers (e.g., 300) are invalid; use 300px. Percentages are not allowed in sizes. Valid units include: px, em, rem, vw, vh, and the full set listed by the validator (e.g., svw, lvh, cm, etc.).

Common pitfalls:

Missing units: sizes="(min-width: 768px) 600, 100vw" → 600 must be 600px.
Using %: sizes="50%" is invalid; use a viewport unit like 50vw.
Malformed list: values must be comma-separated media conditions followed by a length, ending with a fallback length.
Putting sizes on <source> is valid, but syntax must match the same rules as on <img>.

Reference keywords:

sizes
srcset
media (on <source>)
CSS length units

Valid pattern

With conditions: sizes="(media-condition) length, ... , fallback-length"
Without conditions: sizes="fallback-length"

HTML examples

Fix missing units

Before (invalid):

<picture>
  <source
    type="image/webp"
    srcset="hero-800.webp 800w, hero-1200.webp 1200w"
    sizes="(min-width: 900px) 800, 100vw">
  <img
    src="hero-800.jpg"
    srcset="hero-800.jpg 800w, hero-1200.jpg 1200w"
    sizes="(min-width: 900px) 800, 100vw"
    alt="Hero image">
</picture>

After (valid):

<picture>
  <source
    type="image/webp"
    srcset="hero-800.webp 800w, hero-1200.webp 1200w"
    sizes="(min-width: 900px) 800px, 100vw">
  <img
    src="hero-800.jpg"
    srcset="hero-800.jpg 800w, hero-1200.jpg 1200w"
    sizes="(min-width: 900px) 800px, 100vw"
    alt="Hero image">
</picture>

Replace percentage with viewport units

Before (invalid):

<picture>
  <source
    media="(min-width: 600px)"
    srcset="card-600.webp 600w, card-900.webp 900w"
    sizes="50%">
  <img
    src="card-600.jpg"
    srcset="card-600.jpg 600w, card-900.jpg 900w"
    sizes="50%"
    alt="Product card">
</picture>

After (valid):

<picture>
  <source
    media="(min-width: 600px)"
    srcset="card-600.webp 600w, card-900.webp 900w"
    sizes="50vw">
  <img
    src="card-600.jpg"
    srcset="card-600.jpg 600w, card-900.jpg 900w"
    sizes="50vw"
    alt="Product card">
</picture>

Clean, minimal example with multiple slots

<picture>
  <source
    type="image/avif"
    srcset="banner-480.avif 480w, banner-960.avif 960w, banner-1440.avif 1440w"
    sizes="(min-width: 1200px) 960px, (min-width: 600px) 80vw, 100vw">
  <img
    src="banner-480.jpg"
    srcset="banner-480.jpg 480w, banner-960.jpg 960w, banner-1440.jpg 1440w"
    sizes="(min-width: 1200px) 960px, (min-width: 600px) 80vw, 100vw"
    alt="Responsive banner">
</picture>

If the validator reports “found Y at Z,” inspect the exact character at position Z for a missing unit, stray %, or malformed comma/whitespace, then replace with a valid CSS length unit.

Learn more:

Last reviewed: August 08, 2025

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Reading time

1 min read

Category

HTML Validation

Engine

W3C Validator

Last reviewed

August 08, 2025

Resources

Related W3C Validator Issues

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 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”: 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">

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

sizes only accepts a valid media condition list (e.g., (min-width: 600px) 50vw, 100vw); malformed media conditions or missing length values trigger this parse error.

Detailed explanation

Element: source in a picture or audio/video context. For responsive images, use attributes srcset, optional media, and optional sizes.
Attribute: sizes applies only when srcset contains width descriptors (e.g., 320w, 640w). It is a comma-separated list of:
- One or more media conditions followed by a length (e.g., (min-width: 800px) 50vw)
- A final fallback length with no media condition (e.g., 100vw)

Common parse errors:

Missing parentheses around conditions: min-width: 600px → (min-width: 600px)
Using invalid units or tokens: 50 (needs a length like 50vw or 50rem)
Missing length after a condition: (min-width: 800px)
Trailing commas or malformed separators
Using sizes while srcset uses pixel density descriptors (1x, 2x); sizes is ignored for x-descriptors and may confuse validators
Valid media condition syntax mirrors CSS media queries without the @media keyword. Use logical operators and, or, not per CSS Media Queries spec, and ensure spaces and colons are correct.

HTML examples

Correct responsive image with width descriptors and sizes

<picture>
<source
  type="image/webp"
  srcset="hero-480.webp 480w, hero-800.webp 800w, hero-1200.webp 1200w"
  sizes="(min-width: 1200px) 1200px, (min-width: 800px) 800px, 100vw">
<img
  src="hero-800.jpg"
  srcset="hero-480.jpg 480w, hero-800.jpg 800w, hero-1200.jpg 1200w"
  sizes="(min-width: 1200px) 1200px, (min-width: 800px) 800px, 100vw"
  alt="Hero image">
</picture>

Using viewport width units for simpler sizing

<img
src="photo-640.jpg"
srcset="photo-320.jpg 320w, photo-640.jpg 640w, photo-1024.jpg 1024w"
sizes="(min-width: 900px) 50vw, 100vw"
alt="Sample photo">

Correct when using pixel density descriptors (omit sizes)

<img
src="avatar@1x.jpg"
srcset="avatar@1x.jpg 1x, avatar@2x.jpg 2x"
alt="User avatar">

Fixing common mistakes

Add parentheses and a length:

<img
src="img-800.jpg"
srcset="img-480.jpg 480w, img-800.jpg 800w"
sizes="(min-width: 600px) 50vw, 100vw"
alt="Example">

Remove stray comma and ensure final fallback:

<img
src="banner-800.jpg"
srcset="banner-400.jpg 400w, banner-800.jpg 800w"
sizes="(min-width: 700px) 60vw, 100vw"
alt="Banner">

Minimal valid document example

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Valid sizes Attribute</title>
</head>
<body>
  <img
    src="pic-800.jpg"
    srcset="pic-400.jpg 400w, pic-800.jpg 800w"
    sizes="(min-width: 600px) 50vw, 100vw"
    alt="Picture">
</body>
</html>

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.

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