The W3C specifications define the valid CSS properties. Only the official and browser-specific properties should be used to get the expected impact in the final rendering.

This rule ignores:

- $sass, @less, and var(--custom-property) variable syntaxes.

- vendor-prefixed properties (e.g., -moz-align-self, -webkit-align-self).

Noncompliant Code Example

a {
  colour: blue; /* Noncompliant; colour is not part of the specifications */
}

Compliant Solution

a {
  color: blue;
}