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 “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.

Learn more:

Related W3C validator issues

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

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 “src” on element “img”: Backslash ("\") used as path segment delimiter.

This error message indicates that there is a backslash (\) used in a URL, which is not a valid character for URL paths.

You’ll need to replace the backslashes with forward slashes (/) in the URL path segments.

Here’s an example of a correct img tag using a valid URL path:

<img src="https://example.com/img/small/photo.png" alt="example image">

Also, make sure that the URL is correct and that the image file actually exists in the specified location.

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.

When the “srcset” attribute has any image candidate string with a width descriptor, the “sizes” attribute must also be present.

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.

Bad start tag in “img” in “noscript” in “head”.

A start tag for <img> has been found inside a <noscript> section within the <head>, where it’s not allowed. Consider moving it to the <body> section.

The HTML <noscript> element defines a section of HTML to be inserted if a script type on the page is unsupported or if scripting is currently turned off in the browser.

When JavaScript is disabled, the content inside <noscript> will be used instead, so this content must fit within its parent section. As an <img> tag is not allowed inside <head>, this will raise an issue. Instead, consider moving the <noscript> part to the <body> section.

This issue is often related to 3rd party tracking pixels like the Facebook or LinkedIn conversion tracking pixels. For example, the Facebook pixel instructions tell you to insert it like this:

<html>
  <head>
    <script>
       ...some script...
    </script>
    <noscript>
       <img src="..." />
     </noscript>
  </head>
  <body>
     ...
  </body>
</html>

Instead, consider moving the <noscript> part inside the <body>, where the <img> makes sense to be inserted:

<html>
  <head>
    <script>
       ...some script...
    </script>
  </head>
  <body>
     ...
     <noscript>
       <img src="..." />
     </noscript>
  </body>
</html>