Misunderstanding in Media Handler Usage Documentation
From Documentation: http://wcm.io/handler/media/usage.html#Responsive_Images
You can customize the markup that is generated by subclassing ResponsiveImageMediaMarkupBuilder and bring in your own logic.
I try to subclass ResponsiveImageMediaMarkupBuilder but it is not possible, all methods in this Builder are final and can not be overridden. please update Documentation or remove final declaration in methods.
you're right! - documentation is updated.