WordPress has replaced PHP with web technologies and a RESTful back-end API for the administration console.
The new interface is a single page application with a responsive design adapting to screens of various sizes. According to this feature comparison table, Calypso is fast with near instant page loads, instant publishing, in-page previews for multiple targets, and changes are applied in real time without reloading.
Automattic has open sourced Calypso, but they still have “a ton to figure out around plugins, extensibility, contributions, Windows and Linux releases, API speed, localization, and harmonizing the WordPress.com API and WP-API so it can work with core WordPress,” according to Mullenweg. Also, they leave it to the community to decide the inclusion of Calypso in the Core WordPress to replace or not the current admin interface.
WordPress is currently used by a quarter of world’s CMSes, according to W3Techs, up from 13% as it was in 2011.