at-rule-descriptor-value-no-unknown

Disallow unknown values for descriptors within at-rules.

@counter-style foo {
  system: unknown;
/**       ↑
 * Values like this */
}

This rule considers descriptors and values defined in the CSS Specifications, up to and including Editor's Drafts, to be known.

You can filter the CSSTree Syntax Reference to find out what values are valid for a descriptor of an at-rule.

The message secondary option can accept the arguments of this rule.

This rule checks descriptor values within at-rules. You can use declaration-property-value-no-unknown to disallow unknown values for properties within declarations, and at-rule-descriptor-no-unknown to disallow unknown descriptors for at-rules.

Prior art:

Options

The following patterns are considered problems:

@counter-style foo {
  system: unknown;
}

@property --foo {
  syntax: unknown;
}

The following patterns are not considered problems:

@counter-style foo {
  system: numeric;
}

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