Why is this an issue?

The prelude of an at-rule is the content between the rule name and its block or semicolon. It must follow the syntax defined for that specific at-rule.

When the prelude is invalid, the browser ignores the whole at-rule. This makes the stylesheet behave differently from what the code suggests and can silently disable declarations such as custom property registrations or font palette definitions.

Noncompliant code example

@property foo {}
@font-palette-values foo {}

Compliant solution

@property --foo {}
@font-palette-values --foo {}

Resources

Documentation