Bad value X for attribute “sizes” on element “img”: Bad CSS number token: Expected a minus sign or a digit but saw “"” instead at Y.

The value of the sizes attribute on an <img> starts with a literal " character, which can't begin a media condition or a CSS length.

sizes holds a comma-separated list of source sizes. Each one is an optional media condition followed by a length, as in (max-width: 600px) 100vw, and the last one is usually a plain length such as 600px. Lazy-loaded images can also start the list with auto. The quotes around an attribute value are HTML syntax, not part of the value. When an extra " ends up inside it, the validator tries to read the entry as a length, finds the quote where it expects a digit or a minus sign, and rejects the value.

The stray quote usually comes from one of two mistakes. In the first, the quotes around the value were HTML-encoded as ", often by a plugin or template that escaped markup a second time. " can't delimit an attribute, so the parser reads sizes="(max-width: as an unquoted value that stops at the first space. The rest of the list turns into stray attributes such as 600px) and 100vw,, which the validator reports separately, and the bad value in this message is only the first word, such as "(max-width: or "auto,. In the second, the value is wrapped in two sets of quotes, as in sizes='"100vw"', and the whole list is the bad value.

Browsers drop a sizes value they can't parse and fall back to 100vw. They then pick the srcset candidate meant for a full-width layout, which is often a larger file than the image slot needs.

The cause rarely affects sizes alone. If width, height or srcset on the same <img> also report a stray ", fix the code that writes the tag instead of editing each attribute.

Invalid example

<img
  src="logo-600.png"
  alt="Company logo"
  srcset="logo-600.png 600w, logo-1200.png 1200w"
  sizes=&quot;(max-width: 600px) 100vw, 600px&quot;>

Valid example

<img
  src="logo-600.png"
  alt="Company logo"
  srcset="logo-600.png 600w, logo-1200.png 1200w"
  sizes="(max-width: 600px) 100vw, 600px">

Learn more:

MDN Reference

MDN Web Docs: The Image Embed element

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MDN Reference

MDN Web Docs: HTMLImageElement sizes property

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