The :optional pseudo-class matches:
- button  elements
- input elements that are not required
- select elements
- textarea elements that do not have a required attribute

I see two possible behavior for :optional:
 1. it considers every "interactive" elements the user can fill which are not required ;
 2. it considers only elements which can be required but don't have the required attribute specified.

At the moment, the optional pseudo-class doesn't seem to follow any of this rule. Indeed, "button" isn't an element the user can fill so, show it as "optional" sounds weird. The same way, "input elements that are not required" should be changed to "input elements for which the required attribute applies but without the required attribute specified" so button, hidden, image, reset and submit input types don't match the rule.

The difference between behavior 1 and behavior 2 is the the select element and I'm wondering why it can't be required. It sounds reasonable to let it matches :optional but it should be great to have it matches :required too. See bug 9625.

To summarize, button elements should not match the :optional pseudo-class nor input elements for which the required attribute doesn't apply.