CSS: “background”: only “0” can be a “length”. You must put a unit after your number.

A position in the background shorthand is a plain number like 10, and CSS only accepts a number without a unit when it is 0.

In background: url(banner.png) no-repeat 10 20, the two numbers set background-position, which takes lengths such as 10px or 1.5em, percentages, or keywords like left and center. CSS has no default unit, so 10 alone is not a length. Browsers ignore the whole declaration, and the element loses its background color and image along with the position.

The bare number often comes from a template or CMS setting that prints an offset without px, or from a value copied out of a design tool. Add the unit you meant, or switch to a percentage or keyword. 0 is the one number that is valid on its own, because zero is the same length in every unit.

The validator words two close cases differently. A unitless size after the slash, as in center / 100 auto, is reported as a "unit" error instead of a "length" one. A unitless number that comes second, after a valid first position such as 10px 20, is reported as not being a background value. Adding the unit fixes both.

Invalid example

<div style="background: url(banner.png) no-repeat 10 20;">
  <p>Summer sale</p>
</div>

Valid example

<div style="background: url(banner.png) no-repeat 10px 20px;">
  <p>Summer sale</p>
</div>

Learn more:

MDN Reference

MDN Web Docs: background

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MDN Reference

MDN Web Docs: background-position

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MDN Reference

MDN Web Docs: <length>

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