HTML Checking for Large Sites
Rocket Validator automatically scans your sites for accessibility issues using the W3C Validator,
hosted on our own servers and integrated into our web crawler.
The “sizes” attribute may be specified only if the “srcset” attribute is also present.
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.