CSS: “background”: X is an incorrect URL.

The url() in a background value starts with a scheme the CSS validator doesn't recognize, so it can't read the value as a URL.

The validator accepts relative paths such as images/arrow.png or /images/arrow.png, and absolute URLs with common schemes such as https: and data:. It treats everything before the first colon as the scheme, and if it doesn't know that scheme, it reports the whole url() as incorrect. Quotes don't change this: url("myapp://icons/arrow.png") gets the same error.

The usual cause is a placeholder that a build tool or CMS was supposed to replace with a real path, such as url(myapp://icons/arrow.png). A browser can't resolve that scheme either, so the image never loads. Local file paths cause it too: in url(C:/images/arrow.png), the validator takes the drive letter C as the scheme, and visitors' browsers couldn't open a file on your disk anyway.

Look at the CSS the browser receives, not the template. Point url() at the file's real location with a relative path or a full https:// URL, and fix the step that should have replaced the placeholder.

Invalid example

<div style="background: #f5f5f5 url(myapp://icons/arrow.png) no-repeat right center;">
  <p>Next page</p>
</div>

Valid example

<div style="background: #f5f5f5 url(/images/arrow.png) no-repeat right center;">
  <p>Next page</p>
</div>

Learn more:

MDN Reference

MDN Web Docs: background

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MDN Reference

MDN Web Docs: url()

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MDN Reference

MDN Web Docs: URI schemes

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