HTML Guide

The W3C validator error you are seeing indicates that you’ve used the sizes attribute on an <img> element without providing a srcset attribute. According to HTML specifications, the sizes attribute is only meaningful when used alongside the srcset attribute.

The srcset attribute allows you to specify a list of possible image files for the browser to choose from based on conditions like the screen size or resolution. The sizes attribute helps the browser understand what size the image should be under specific conditions defined in srcset.

How to Fix the Issue

There are two options, adding or removing attributes.

1. Adding a srcset Attribute: If you need responsive images, make sure to include the srcset attribute in your <img> element.

2. Removing the sizes Attribute (if not necessary): If responsive images are not required and you’re not using srcset, simply remove the sizes attribute.

Example

Incorrect Markup:

<img src="image.jpg" sizes="(max-width: 600px) 480px, 800px" alt="Example Image">

Correct Markup with srcset:

<img src="image.jpg" 
     srcset="image-480w.jpg 480w, image-800w.jpg 800w" 
     sizes="(max-width: 600px) 480px, 800px" 
     alt="Example Image">

In this correct example, the srcset attribute provides two different images (image-480w.jpg and image-800w.jpg) and their corresponding widths (480w and 800w). The sizes attribute tells the browser to use the 480 pixels wide image if the viewport is 600 pixels or less, otherwise, it will use the full 800 pixels.

Correct Markup without sizes and srcset:

<img src="image.jpg" alt="Example Image">

If you don’t need responsive images, simply remove the sizes attribute to fix the issue.

Summary

Ensure that whenever you use the sizes attribute, you also include a srcset attribute. This makes sure that the browser can effectively make decisions about which image size to load in different scenarios.

Check the HTMLImageElement.srcset guide to learn about the correct usage of the srcset and sizes attributes.

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.

The sizes attribute is used to complement the srcset attribute on an <img> tag for responsive images. When this attribute is present, all image candidates must specify its width.

The srcset property on img elements, when used, requires at least one value. This property is a string which identifies one or more image candidate strings, separated using commas (,) each specifying image resources to use under given circumstances.

Each image candidate string contains an image URL and an optional width or pixel density descriptor that indicates the conditions under which that candidate should be used instead of the image specified by the src property.

Example:

<img
    src="/img/cat-200px.png"
    alt="Cat"
    srcset="
      /img/cat-200px.png 1x,
      /img/cat-400px.png 2x
    ">

The srcset property on source elements, when used, requires at least one value.

The <source> HTML element specifies multiple media resources for the <picture>, the <audio> element, or the <video> element. It is a void element, meaning that it has no content and does not have a closing tag. It is commonly used to offer the same media content in multiple file formats in order to provide compatibility with a broad range of browsers given their differing support for image file formats and media file formats.

The srcset attribute is required if the source element’s parent is a <picture> element, but not allowed if the source element’s parent is an <audio> or <video> element.

The sizes attribute is used to complement the srcset attribute on an <img> tag for responsive images. When this attribute is present, all image candidates must specify its width.