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

NaN in JavaScript stands for “Not a Number” and appears when a numeric operation fails — for example, parsing a non-numeric string with parseFloat(), dividing 0 by 0, or referencing an undefined variable in arithmetic. When this NaN value gets concatenated with a unit string like "rem", the result is "NaNrem", which is meaningless to CSS. The browser cannot interpret it, and the W3C validator flags it as an invalid letter-spacing value.

This issue almost always originates from dynamically generated styles — either through JavaScript that sets inline styles, a server-side template that computes CSS values, or a CSS-in-JS library. The rendered HTML ends up containing something like style="letter-spacing: NaNrem", which the validator rightly rejects.

Beyond validation, this is a practical problem: the browser will ignore the invalid declaration entirely, so whatever letter-spacing you intended won’t be applied. Your layout may look different than expected, and the invalid value in the markup signals a bug in your code that could affect other computed styles too.

How to Fix It

1. Trace the source of the value. Search your codebase for where letter-spacing is set. If it’s an inline style, look at the JavaScript or server-side code that generates it.

2. Validate the number before using it. In JavaScript, use Number.isNaN() or isFinite() to check that a computed value is valid before applying it.

3. Provide a sensible fallback. If the calculation might fail, default to a known-good value like 0 or normal.

4. Fix the root cause. Determine why the calculation produces NaN — common causes include missing data attributes, undefined variables, or parsing non-numeric strings.

Examples

❌ Invalid: NaN concatenated with a unit

This is what the rendered HTML looks like when the bug occurs:

<p style="letter-spacing: NaNrem">Spaced text</p>

The JavaScript that likely produced it:

// Bug: getAttribute returns null if data-spacing is missing
let spacing = parseFloat(element.getAttribute('data-spacing'));
element.style.letterSpacing = spacing + 'rem'; // "NaNrem" if attribute is missing

✅ Fixed: Validate the value before applying it

let raw = parseFloat(element.getAttribute('data-spacing'));
let spacing = Number.isFinite(raw) ? raw : 0.1; // fallback to 0.1
element.style.letterSpacing = spacing + 'rem';

This produces valid inline CSS:

<p style="letter-spacing: 0.1rem">Spaced text</p>

✅ Fixed: Using a static CSS value

If the letter-spacing doesn’t need to be dynamic, the simplest fix is to use a plain CSS rule instead of computing it in JavaScript:

<style>
  .spaced-text {
    letter-spacing: 0.1rem;
  }
</style>
<p class="spaced-text">Spaced text</p>

✅ Fixed: Server-side template with a guard

If a server-side template generates the style, add a check before rendering:

<!-- Pseudocode for a template engine -->

<!-- Only output the style attribute if the value is a valid number -->

<p style="letter-spacing: 0.05rem">Spaced text</p>

Ensure your template logic verifies the value is numeric before injecting it into the markup. If the value is missing or invalid, either omit the style attribute entirely or use a safe default.

Valid letter-spacing values for reference

The letter-spacing property accepts:

• The keyword normal (default, lets the browser decide)
• Any valid CSS length: 0.1rem, 1px, 0.05em, 2px, etc.
• 0 (no extra spacing)

The numeric part must always be a real number — NaN, Infinity, and empty strings are never valid.

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