Pseudo class for state: should it be :checked?

[domenic]

Related to #2. I see a few options:

• Use :checked and :not(:checked)
• Use :state(on), and then either :state(off) or :not(:state(on))
• If we go with the simple attribute reflection, we can use attribute selectors: [checked] and :not([checked]) or [on] and :not([on]).