Why is this an issue?

At-rules such as @counter-style and @property accept only descriptor values defined by CSS specifications. When a descriptor is assigned an unknown value, browsers ignore it and the at-rule may not behave as intended.

This rule applies only to plain CSS. It does not raise issues in SCSS, Sass, or Less files.

Noncompliant code example

@counter-style chapters {
  system: unknown;
}

@property --accent {
  syntax: foo();
}

Compliant solution

@counter-style chapters {
  system: numeric;
}

@property --accent {
  syntax: "<color>";
}

Resources

Documentation