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The “sizes” attribute may be specified only if the “srcset” attribute is also present.

sizes
srcset

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.

Related W3C validator issues

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

srcset
sizes
img

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