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. -moz-align-self, -webkit-align-self).
a {
colour: blue; /* Noncompliant; colour is not part of the specifications */
}
a {
color: blue;
}