[VE][html5] spurious "must have child option" when all options in optgroups

I got the following error when validating my document. The HTML 4.01 
spec says that the select element must contain an option, but it does 
not say that it must be a child of the select element. The validator 
throws the message when all options are inside optgroups, and I think 
that should not be an error.

Error [html5]: " A select element with a required attribute and without 
a multiple attribute, and whose size is 1, must have a child option 
element."