Antonio is a 'refined' version of the Anton Font. Anton is a single weight web font, designed specifically for larger display, headline and 'banner' use (see Google's PR for the Chromebook notebooks that used Anton, big and bright).
Antonio extends the Anton design to include more weights and introduces refinements to the design that makes it also suitable for use in smaller headings, menus and 'buttons' etc.
To contribute, see github.com/googlefonts/antonioFont