CSS: “letter-spacing”: X is not a “letter-spacing” value.

letter-spacing takes normal or a length such as 0.05em, and the validator rejects anything else, including percentages like 1%.

CSS Text Module Level 4 adds percentages to letter-spacing and resolves them against the font size, so 1% is one hundredth of the font size. Only recent browser versions support this, and the validator's CSS checker doesn't recognize it. A browser without support ignores the declaration, and the text keeps whatever spacing it inherits.

The em unit gives the same result in every browser. 1em is the element's font size, so 1% becomes 0.01em and 5% becomes 0.05em. The spacing still scales with the text.

Made-up keywords such as wide trigger the same message. Use normal to reset the spacing, 0 for no extra space, or a length like 0.05em or 1px. Negative lengths are allowed and pull the letters closer together.

Invalid example

<h1 style="letter-spacing: 1%;">Upcoming events</h1>

Valid example

<h1 style="letter-spacing: 0.01em;">Upcoming events</h1>

Learn more:

MDN Reference

MDN Web Docs: letter-spacing

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MDN Reference

MDN Web Docs: <length>

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