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ValidateHTML

Missing Unit on CSS Value

In CSS, numeric values (except 0) must have a unit. Writing 'margin: 10' instead of 'margin: 10px' is invalid. The only exceptions are unitless values like line-height, opacity, flex-grow, z-index, and font-weight, which accept plain numbers by design.

Why It Matters

A missing unit causes the entire declaration to be ignored. The property reverts to its default or inherited value, which can cause layout shifts, spacing issues, or invisible elements.

Common Causes

  • Copying a raw number from a design tool that shows sizes without units.
  • Assuming a length property accepts a bare number the way line-height or opacity does.
  • Dropping the unit on one value inside a shorthand, such as padding: 10px 15.

Code Examples

Bad CSS
div {
  margin: 20;          /* needs a unit */
  padding: 10 15;      /* both need units */
  font-size: 16;       /* needs a unit */
  border: 1 solid red; /* width needs unit */
}
Good CSS
div {
  margin: 20px;
  padding: 10px 15px;
  font-size: 16px;
  border: 1px solid red;
}

How to Fix

  • 1Always add a unit to non-zero numeric values: px, em, rem, %, vh, vw.
  • 2Exception: 0 never needs a unit. 'margin: 0' is correct (not 'margin: 0px').
  • 3Properties that accept unitless numbers: line-height, opacity, z-index, flex-grow, flex-shrink, font-weight, order.
  • 4When in doubt, use px for fixed sizes, rem for scalable sizes, % for relative sizes.

Frequently Asked Questions

Which CSS properties accept a number with no unit?
Unitless numbers are valid for line-height, opacity, z-index, flex-grow, flex-shrink, order, and font-weight. Length properties like margin, padding, width, and font-size always require a unit on non-zero values.
Does zero need a unit?
No. Zero is the one number that is valid with or without a unit, because zero length is the same in every unit. Writing margin: 0 is correct and slightly shorter than margin: 0px.
Why is a unitless line-height fine but a unitless margin is not?
line-height is defined to accept a plain number as a multiplier of the font size, which scales correctly with inheritance. margin is a length, and lengths have no meaning without a unit, so a bare number is rejected.

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