HTML Guide for srcset

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 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.

Ensure all descriptors in the srcset have positive widths greater than zero. In the srcset attribute, each source candidate consists of a URL followed by a width descriptor, which should be a positive integer followed by w.

The issue arises from using 0w, which is not valid as width descriptors must be positive numbers. The srcset attribute allows browsers to choose the appropriate image from different candidates based on device resolution and screen size, so specifying a meaningful width is important.

Here’s how you can fix it:

1. Verify that each candidate in the srcset includes a valid URL and a width descriptor greater than zero.
2. Decide the relevant width for your images and update the descriptors accordingly.

Here’s a corrected example of how to use the srcset attribute:

<picture>
  <source 
    srcset="/images/icon_small_1x.png 300w,
            /images/icon_small_2x.png 600w" 
    media="(max-width: 600px)">
  <img src="/images/icon_fallback.png" 
       alt="App Logo">
</picture>

In this example, each image in srcset has a positive width descriptor: 300w and 600w. This ensures that the browser can make an appropriate choice based on the current viewport and resolution, adhering to the W3C HTML standards.

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.

The sizes attribute is used to complement the srcset attribute on an <img> tag for responsive images, therefore it’s not valid if srcset is missing.

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.