Disallow invalid hex colors.
a { color: #y3 }
/** ↑
* This hex color */
Longhand hex colors can be either 6 or 8 (with alpha channel) hexadecimal characters. And their shorthand variants are 3 and 4 characters respectively.
The message
secondary option can accept the arguments of this rule.
The following patterns are considered problems:
a { color: #00; }
a { color: #fff1az; }
a { color: #12345aa; }
The following patterns are not considered problems:
a { color: #000; }
a { color: #000f; }
a { color: #fff1a0; }
a { color: #123450aa; }