Why is this an issue?

At-rules such as @counter-style and @property define a fixed set of descriptors. Browsers silently ignore unknown descriptors, so a typo can change the at-rule behavior or make it ineffective.

This rule only applies to plain CSS files. CSS-like syntaxes such as SCSS and Less can legitimately use descriptors that are outside the CSS specifications.

Noncompliant code example

@counter-style foo {
  sysem: cyclic;
}

@property --brand-color {
  unknown-descriptor: "<color>";
  inherits: false;
  initial-value: red;
}

Compliant solution

@counter-style foo {
  system: cyclic;
}

@property --brand-color {
  syntax: "<color>";
  inherits: false;
  initial-value: red;
}

Resources

Documentation