Bad value X for attribute “decoding” on element “img”.

The decoding attribute of an <img> is set to something other than sync, async or auto.

decoding is a hint about when the browser should decode the image. async says the image can be decoded without delaying the rest of the content, sync asks for it to be decoded and shown together with the content around it, and auto, the default, leaves the choice to the browser. The value has to be one of those three lowercase keywords with nothing around it. A typo such as asynch, a keyword that belongs to another attribute such as lazy (that one goes in loading), uppercase ASYNC and an empty value all trigger this error.

When the reported value includes quotes, as in "async", the quotes are inside the value itself. That usually happens when 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 decoding="async" as an unquoted value and decodes both references into literal quotes. Wrapping the value in two sets of quotes, as in decoding='"async"', gives the same result. A value with only a trailing quote, such as async", means the opening quote is missing, and the validator also reports “"” in an unquoted attribute value.

Browsers treat an invalid value as auto, so the image still loads and displays, but the hint is lost. If you don't need a particular decoding mode, remove the attribute.

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

Invalid example

<img src="photo.jpg" alt="A red bicycle" width="600" height="400" decoding=&quot;async&quot;>
<img src="photo.jpg" alt="A red bicycle" width="600" height="400" decoding="asynch">

Valid example

<img src="photo.jpg" alt="A red bicycle" width="600" height="400" decoding="async">

Learn more:

MDN Reference

MDN Web Docs: The image embed element

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MDN Reference

MDN Web Docs: HTMLImageElement decoding property

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WHATWG Spec

HTML Standard: Embedded content

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