HTML Guide

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 Y at Z.

sizes
img

Invalid sizes syntax in the <img> sizes attribute causes the validator error; each size value must be a valid CSS length with units and optional media condition.

Detailed explanation

The sizes attribute on responsive images (used with srcset) tells the browser how wide the image will be in CSS pixels under certain media conditions so it can choose the best candidate from srcset. It must be a comma-separated list of one or more size descriptors:

Optional media condition followed by a length: (<media-condition>) <length>
Or a final fallback length without media condition: <length>

Valid length units include: 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. Percentages are not allowed in sizes. Bare numbers without units are invalid. The media condition must be valid per CSS Media Queries (e.g., (max-width: 600px)), and each list item must have exactly one length value.

Common mistakes that trigger this error:

Missing units: sizes="(max-width: 600px) 100" → must be 100px or another unit.
Using %: sizes="(max-width: 600px) 100%" → percentages are invalid in sizes.
Multiple lengths per item: sizes="(min-width: 800px) 50vw 400px" → only one length per item.
Typos in units: 100 pxx, 100vws.
Missing fallback: while not strictly required if all conditions cover all cases, providing a final fallback length avoids unexpected behavior.

Ensure sizes pairs correctly with srcset; srcset provides image candidates with width descriptors (w), and sizes expresses the slot width the image will occupy.

HTML examples

Correct usage with media conditions and fallback

<img
  src="img-400.jpg"
  srcset="img-400.jpg 400w, img-800.jpg 800w, img-1200.jpg 1200w"
  sizes="(max-width: 600px) 100vw, (max-width: 900px) 50vw, 33vw"
  alt="Sample image">

Fixing missing units (was 100 -> 100px)

<img
  src="avatar-200.jpg"
  srcset="avatar-200.jpg 200w, avatar-400.jpg 400w"
  sizes="(max-width: 480px) 100vw, 200px"
  alt="User avatar">

Avoiding invalid percentages (use `vw` instead)

<img
  src="banner-800.jpg"
  srcset="banner-800.jpg 800w, banner-1600.jpg 1600w"
  sizes="(max-width: 700px) 100vw, 80vw"
  alt="Promotional banner">

Single unconditional size (no media condition)

<img
  src="thumb-320.jpg"
  srcset="thumb-320.jpg 320w, thumb-640.jpg 640w"
  sizes="320px"
  alt="Thumbnail">

Correct `srcset` with `sizes` alignment

<img
  src="photo-640.jpg"
  srcset="photo-640.jpg 640w, photo-960.jpg 960w, photo-1280.jpg 1280w"
  sizes="(min-width: 1200px) 800px, (min-width: 800px) 60vw, 90vw"
  alt="Landscape photo">

If the validator pinpoints a position (Z), check that token for a missing or invalid unit, extra tokens after the length, or malformed media condition.

Learn more:

Related W3C validator issues

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 “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 “img”: Empty source size at Y.

sizes
img

The sizes attribute contains an empty source size, probably due to a trailing comma.

According to the HTML standard, the sizes attribute on the img element must contain a valid source size list. Each size is separated by a comma, and there must not be a trailing comma because that would create an empty entry, which is not allowed.

An example of an invalid sizes attribute:

<img
  src="image.jpg"
  alt=""
  sizes="(min-width: 2200px) 100vw, (min-width: 856px) 461px, (min-width: 784px) 615px, "
  srcset="image-2200.jpg 2200w, image-856.jpg 856w">

To fix the validation error, remove the trailing comma so the size list does not end with an empty value:

<img
  src="image.jpg"
  alt=""
  sizes="(min-width: 2200px) 100vw, (min-width: 856px) 461px, (min-width: 784px) 615px"
  srcset="image-2200.jpg 2200w, image-856.jpg 856w">

Each value in the sizes attribute should be a valid media condition and source size, separated only by commas, with no trailing or empty values.

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.

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.

An “img” element must have an “alt” attribute, except under certain conditions. For details, consult guidance on providing text alternatives for images.

<img> tags, used to include images on a document, require an alt attribute to describe the contents of the image. This is essential for users that cannot see the image (like screen reader users), or as an alternate text when the image cannot be displayed. Example:

<img src="photo.jpg" alt="Person holding a cat" />

An “img” element which has an “alt” attribute whose value is the empty string must not have a “role” attribute.

Detailed explanation

HTML examples

Correct usage with media conditions and fallback

Fixing missing units (was 100 -> 100px)

Avoiding invalid percentages (use vw instead)

Single unconditional size (no media condition)

Correct srcset with sizes alignment

Related W3C validator issues

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)

Example

Detailed explanation:

Common parse errors:

HTML examples:

Avoiding invalid percentages (use `vw` instead)

Correct `srcset` with `sizes` alignment