But how can I hide the multiplicity on a property with multiplicity of "1" or for each property with unspecified multiplicity with css?

I dont want to add this information to the diagram if a property has a multiplicity of "1" or "unspecified".

Which css class do I have to use?

Something like:

Property > Label[kind=Multiplicity][data-multiplicity="1"], 
Property > Label[kind=Multiplicity][data-multiplicity="unspecified"]{
visible: false;
}