Constrains of the value propety of Controls

Currently, no DOM specification explicitly defines behaviour and  
restrictions when setting the value of the value property for elements  
part of forms, like select, option, and input. That's however, work that's  
being done at WHATWG.
Yet, one single case persists:
  - the value property of a HMTLSelectElement is a limited domain, formed  
by the value of the select's child option elements value property.

Is this a valid assertion ?
What is expected to happen when a script tries to set the value of the  
value property of a select element, value which doesn't exist among it's  
children ? Should the value remain unchanged ? An error thrown ?

thank you.