HTML <select> required Attribute not working in Visualforce?

I'm trying to implement a form with a required drop-down list on a VF page, as described in this example. If you click on the link above and run the sample, it works fine but when I try to implement it on a VF page it does not. In my code below I have two fields of type text and select, the validation runs on the text field if it remains blank but not on the select (if the value is set to the default None).

Please advise