HTML Checking for Large Sites

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.

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.

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