Validación HTML: CSS: “line-height”: X is not a “line-height” value.

Acerca de este problema HTML

The line-height CSS property controls the spacing between lines of text within an element. When the validator reports that a value “is not a line-height value,” it means the value you provided doesn’t match any of the accepted formats defined in the CSS specification.

Common mistakes that trigger this error include:

Misspelled keywords — writing norml or Normal instead of normal (CSS keywords are case-insensitive in browsers, but some validators may flag inconsistencies; the real issue is outright misspellings).
Invalid or missing units — using a unit the spec doesn’t support (e.g., line-height: 1.5x;) or accidentally adding a unit to what should be a unitless value (e.g., confusing 1.5 with 1.5 em with a space).
Malformed numbers — typos like 1..5 or 24ppx.
Using unsupported keywords — values like auto, thin, or large are not valid for line-height.
Negative values — line-height does not accept negative numbers.

This matters for standards compliance and predictable rendering. While browsers may silently ignore an invalid line-height value and fall back to a default, this means your intended styling won’t be applied — potentially causing overlapping text, poor readability, or inconsistent layouts across browsers. Fixing validation errors ensures your styles work as intended everywhere.

Valid `line-height` values

Format	Example	Description
Keyword	`normal`	Browser default (typically around 1.2)
Unitless number	`1.5`	Multiplied by the element’s font size — recommended
Length	`24px`, `1.5em`, `2rem`	An absolute or relative length
Percentage	`150%`	Relative to the element’s font size

The unitless number format is generally preferred because it scales properly with inherited font sizes, avoiding unexpected results in nested elements.

Examples

❌ Incorrect: invalid values that trigger the error

<!-- Misspelled keyword -->

<p style="line-height: norml;">Text with an invalid line-height.</p>

<!-- Invalid unit -->

<p style="line-height: 1.5x;">Text with an unrecognized unit.</p>

<!-- Malformed number -->

<p style="line-height: 1..5;">Text with a typo in the number.</p>

<!-- Unsupported keyword -->

<p style="line-height: auto;">Auto is not valid for line-height.</p>

<!-- Negative value -->

<p style="line-height: -1.5;">Negative values are not allowed.</p>

✅ Correct: valid `line-height` values

<!-- Keyword -->

<p style="line-height: normal;">Browser default line height.</p>

<!-- Unitless number (recommended) -->

<p style="line-height: 1.5;">1.5 times the font size.</p>

<!-- Length with px -->

<p style="line-height: 24px;">Fixed 24px line height.</p>

<!-- Length with em -->

<p style="line-height: 1.5em;">1.5em line height.</p>

<!-- Percentage -->

<p style="line-height: 150%;">150% of the font size.</p>

Full document example

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Valid line-height Example</title>
</head>
<body>
  <h1 style="line-height: 1.2;">A Heading with Tight Spacing</h1>
  <p style="line-height: 1.6;">This paragraph uses a unitless line-height of 1.6,
    which is a common choice for body text readability. It scales correctly even
    if child elements have different font sizes.</p>
</body>
</html>

Tip: unitless vs. percentage/length

A unitless line-height and a percentage line-height may look equivalent, but they behave differently with inherited styles. A unitless value is recalculated for each child element based on its own font size, while a percentage or length is computed once on the parent and that fixed value is inherited. For most cases, unitless numbers are the safest choice.