2.2.2 [W3C-XSS] Section 3.8.4, Model Group Validation Rules


The specification states:

 Nothing in the above should be understood as ruling out groups whose 
 {particles} is empty: although no sequence can be ·valid· with respect to 
 such a group whose {compositor} is choice, the empty sequence is ·valid· with 
 respect to empty groups whose {compositor} is sequence or all.


An empty group that has a {compositor} property with a value of choice can be valid.