CSS: “margin”: “"X"” is not a “margin” value.

In CSS, property values such as lengths, percentages, and keywords are written without quotation marks. Quotes in CSS are reserved for specific contexts like content property strings, font family names with spaces, and url() paths. When you wrap a margin value (or any similar CSS property value) in double or single quotes, the CSS parser interprets it as a string literal rather than a set of length or keyword values. Since "0 0 1em 0" is a string and not a valid margin value, the declaration is ignored by browsers and flagged by the W3C validator.

This is a problem for several reasons. First, the style will silently fail — browsers discard CSS declarations they can’t parse, so your intended margins won’t be applied, potentially breaking your layout. Second, it indicates a misunderstanding of CSS syntax that could lead to similar errors in other properties. This mistake commonly occurs when developers confuse HTML attribute quoting rules with CSS value syntax, especially when writing inline style attributes where the attribute value itself is already quoted.

The margin property accepts one to four values, each of which can be a length (e.g., 10px, 1em), a percentage, auto, or a global keyword like inherit. None of these require quotes. The fix is straightforward: remove the quotation marks around the CSS value.

Examples

❌ Incorrect: margin value wrapped in quotes

In a <style> block:

<style>
  .card {
    margin: "0 0 1em 0";
  }
</style>

In an inline style:

<p style="margin: '10px auto'">Hello</p>

Both of these produce the validator error because the CSS parser sees a quoted string instead of valid margin values.

✅ Correct: margin value without quotes

In a <style> block:

<style>
  .card {
    margin: 0 0 1em 0;
  }
</style>

In an inline style:

<p style="margin: 10px auto">Hello</p>

Valid margin value formats

For reference, here are the accepted patterns for the margin property — none of which use quotes:

/* All four sides */
margin: 1em;

/* Vertical | Horizontal */
margin: 5% auto;

/* Top | Horizontal | Bottom */
margin: 1em auto 2em;

/* Top | Right | Bottom | Left */
margin: 2px 1em 0 auto;

/* Global keywords */
margin: inherit;
margin: initial;
margin: unset;

Watch out for inline style quoting confusion

A common source of this mistake is confusion about the quotes used for the HTML style attribute versus the CSS values inside it. The outer quotes delimit the attribute value for HTML — the CSS inside should not have its own quotes around property values:

<!-- ❌ Wrong: extra quotes around the CSS value -->

<div style="margin: '1em'"></div>

<!-- ✅ Correct: only the HTML attribute is quoted -->

<div style="margin: 1em"></div>

This same rule applies to other CSS properties like padding, border, font-size, color, and so on. If you see a similar validator error for any CSS property, check whether you’ve accidentally quoted the value.

Más información:

MDN Reference

MDN: CSS margin

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MDN Reference

CSS values and units

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