They really can hold like anything

:root { 
  --SQL: DELETE * FROM users;
  --all-padding: 10px 20px 30px 40px;
  --booleans: false;
  --math: sqrt(pow(var(--x), 0));
  --gradients: conic-gradient(…);
  --an-icon: url(…);
}

but… the type is 'unknown'