The list of supported CSS properties is growing quite large, and it's very easy to miss a typo.
This rule checks each property name to make sure that it is a known CSS property. All vendor-prefixed properties are ignored because vendors may add in their own properties at any point in time.
a {
clr: red; /* Noncompliant: 'clr' isn't a known property */
}
.mybox {
color: rgb(255,0,51); /* Compliant: 'color' is a known property */
-moz-background-color: 10px; /* Compliant: '-moz-background-color' is a vendor property */
}